Math and the RD and DTR Exams
Math and the RD and DTR Exams
Probably the most asked about topic I receive from students is help with the math used in the many nutrition equations. I thought it would be useful to post an article on what you can expect in your RD or DTR Exam along with some tips to help you conquer (or at least feel more comfortable with) nutrition math problems.
Are you a “mathlete”?
Some people are born to solve math problems. I like to call these individuals mathletes. Mathletes can make almost any situation into an equation to solve. When I swim, I will catch myself not only counting the number of laps in the pool, but then trying to determine what percentage of my workout is complete. This actually helps in the monotony of lap after lap, but that is another story.
Many nutrition students struggle with the abundance of equations required to remember in order to prepare for their registration exams. And who can blame them? Energy and nutrient needs, BMI, employee turnover rate, edible portion and as purchase, FTEs… The list goes on! But in order to pass your exam, you need to illustrate understanding of all the material that can be thrown at you, including math.
What to Expect…
It’s certainly not feasible for the CDR (Commission on Dietetic Registration) to ask you so many math questions on your exam as there are several other areas that need to be covered in order to prove your competency. Throughout the years, I’ve heard back from students, as well as seen on my own RD Exam long ago, that the amount of math questions asked on the exam was very low. I swear on my test, I had only about 10 math questions (less than 10% of the exam), and I was bummed, because I love math!
However, do not take this to the bank. Every test is different. You could very well see twice as many math problems as I did.
Don’t memorize. Familiarize!
No one likes memorizing formulas. Instead, find your weak areas. If you feel very confident in solving problems related to clinical nutrition (tube feeding calculations, protein and energy needs for patients, etc), but have a difficult time with foodservice-type questions, focus your efforts here but still review the clinical equations so you don’t get rusty.
Practice makes perfect!
Once you’ve identified your weak links, get in some repetition work. Take the formulas, add in your own variables, and rework the formula. This will prevent you from memorizing the answer. For example, your study guide has the following question:
Edible portion weight is 25 pounds, and the percent yield is 87%. What amount was purchased (as purchased, or AP)?
The general formula is AP = EP / yield. Now go ahead and change the EP weight and the yield percent. Solve again.
Also, become familiar with the equation written differently, so you can be prepared if this asked for the edible portion weight, or for the yield.
To find edible portion, the same formula is written like: EP = AP x yield. Now add in numbers and solve.
To find the yield percent: yield = EP / AP. Add numbers. Solve.
Taking this approach can help you understand all aspects of the equation. Be ready for whatever is being asked!
The equation is right under your nose!
Many times you can work out the formula simply by reading the question. For example, when the question is asking “what is the cost per portion?”, cost per portion means you are taking the total cost of what is asked and dividing by the number of portions given.
If the variables provided are as follows: food cost was $100, labor cost was $200, and the number of portions served was 60, what is the cost per portion? $100 + $200 = $300 is your total cost, and now divide this by 60 portions served = $5 per portion (cost per portion).
Eliminate the obvious
Sometimes you can eliminate at least one of the choices because the answer makes no sense. Taking the above example, here are your choices:
At first glance, which one would you discard? I would think the least likely choice is A) – I would find it hard to believe that an establishment can have a cost per portion of only 20 cents and charge a high cost to the customer, so I would eliminate this one.
But I can reach each answer provided!
The CDR is not purposely trying to give you anxiety. Yes, many times you can solve for some or all of the answers based on the information provided. The testing organization is trying to see if you truly know how to solve the problem. Do not always stop on the first answer you reach. Here is each of the answers in the above example using data from the question:
A) $0.20 (60 / $300, but this is portion per cost, not what we’re looking for)
B) $1.67 ($100 / 60, this is cost per portion, but only accounting for the food cost per portion, and labor cost is another cost to figure into the equation)
C) $3.33 ($200 / 60, same as choice B, but this accounts for only the labor cost and neglects the food cost)
D) $5.00 ($300 / 60, this is your correct choice as it is cost per portion and accounts for all costs)
Don’t feel obligated to use all of the question data
Again, not CDR trickery. You can very well be presented with information in the question that is just not pertinent to what is being asked. Here’s an example:
A 1.5 cal/ml strength enteral formula is run at 85 ml/hr for 14 hours. What is the total volume of formula provided?
What are you being asked? Total volume. There is no need to use the “1.5 calorie strength formula” information. Simply multiply the tube feeding rate by the total time to infuse: 85 ml per hour x 14 hours = 1,190 ml total volume
Had the question asked you what the total calories are provided, then you would now include the 1.5 data:
85 ml per hour x 14 hours x 1.5 cals per ml = 1,785 calories provided in this regimen