On a science test, the top grade was 42 points higher than the lowest grade. The sum of the two grades was 138. What was the lowest grade?

Answer 1

T-L = 42

T+L= 138

add the two equations to eliminate one variable

T+T + L -L = 42+138

2T = 180

T = 180/2 = 90 for top grade

L = 90-42 = 48 for lowest grade