Billy bought 5 non-fiction books. The original price of each non-fiction book was $25. He was given 10% discount on the first non-fiction book and 20% discount on the rest of the non-fiction books. How much did he pay for the 5 non-fiction books altogether?
Cost of the first non-fiction book after 10% discount in percent
= 100% - 10%
= 90%

Cost of the first non-fiction book after discount
= 90% x 25
= ⁹⁰₁₀₀ x 25
= $22.50

Number of the rest of the non-fiction books that enjoyed 20% discount
= 5 - 1
= 4

Cost of each of the rest of the 4 non-fiction books after 20% discount in percent
= 100% - 20%
= 80%

Cost of each of the rest of the 4 non-fiction books
= 80% x 25
= ⁸⁰₁₀₀ x 25
= $20

Cost of the rest of the 4 non-fiction books that enjoyed 20% discount
= 4 x 20 
= $80

Total cost for the 5 non-fiction books
= 22.50 + 80
= $102.50

Answer(s): $102.50