Answer:
Mr. Mathewson increased the amount of weight he lifted by 12.5%
Step-by-step explanation:
90 = 80 + x% × 80
When you increase an amount A by x%, that means you find x% of A, and add it to A. By the way, the percentage decrease to get back to A is _always_ different.
90 - 80 = (80 + x% × 80) - 80
10 = x% × 80
x% = 10÷80
% means divide by 100 (per means divide, cent means 100)
(x÷100) × 100 = (10÷80) × 100
x = 1000 ÷ 80
= 100 ÷ 8
= 25 ÷ 2 = 12.5
x% = 12.5%
Check: 12.5% of 80 is 0.125 × 80 = 10
You might recognize 0.125 as 1/8.
To increase 80 by 12.5% you add
12.5% of 80 to 80, giving 90. ✔
