Question

An exam has 20 exercises.For every exercise solved correctly you get 8 points and for every exercise solves wrong are taken off 5 points.Ben had 13 points.How many exercises did he solve?

Answer 1

13 exercises

Explanation:

The number of exercises in the exam = 20

The points received for questions solved correctly = 8 points

The points taken off for exercises solved wrong = 5 points

The number of points Ben has = 13 points

Let 'c' represent the number of exercises he solved correctly and let 'w' represent the number of exercises he solved wrong, we have;

0 < c + w ≤ 20...(1)

8·c - 5·w = 13...(2)

Given that 5·w has either a 0 or 5 as the unit digit, and x3 = x8 - 5, such as 13 = 18 - 5, we have that the last digit of 8·c is 8, and the last digit of 5·w is 5

The products of 8 with '8' as the unit are, 8 × 6, 8 × 11, and 8 × 16

Therefore, when c = 6, and w = 7, we get;

8 × 6 - 5 × 7 = 13

The number of exercises Ben solved, c + w = 6 + 7 = 13 < 20

When c = 11, we get;

8 × 11 - 5 × 15 = 13

11 + 15 = 26 > 20

The number of exercises Ben solved = 13 exercises