do the maths
• make a calculation or come to a conclusion based on the relevant facts and figures, typically with the implication that the result is or should be obvious.
• "I have four children and one on the way; you do the maths"
do the math
• make a calculation or come to a conclusion based on the relevant facts and figures, typically with the implication that the result is or should be obvious.
• "diesel prices in my area are approximately 7% more than regular gas, but the mileage is over 20% greater—do the math"
