a student missed 13 problems on a math test and received a grade of 80%. if all problems were of equal value , how many questions were on the test?

Answer 1

13 questions are (100-80%) = 20% of the total number of questions

13/.20 = 13/(1/5) = 13(5) = 65 questions total

Answer 2

Let x be the total number of problems on the test. The grade is calculated by dividing the number of problems correct by the total number of problems. 80% is .8 in decimal form. Observe below how we can solve for x:

(x - 13) / x = .8 Note: x - 13 is the number correct, since we know we got 13 wrong.

x - 13 = .8x

.2x - 13 = 0

.2x = 13

x = 65

65 problems on the test

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