A machine that is 80 percent efficient delivers 400 joules of useful output. How much energy went in?

Correct answer to question 61 (Mechanical Comprehension): 500 joules

Method

Output is 80 percent of input, so input is 400 divided by 0.80, which is 500 joules. The missing 100 joules went to friction and heat. Taking 80 percent of 400 gives 320 and answers the question backwards.