Ken is working this summer as part of a crew on a farm. He earned $8 per hour for the first 10 hours he worked this week. Because of his performance, his crew leader raised his salary to $10 per hour for the rest of the week. Ken saves 90% of his earnings from each week. What is the least number of hours he must work the rest of the week to save at least $270 for the week?

The correct answer is: 22

HINT –  Form an inequality between total earnings and money saved every week
SOLUTION - According to question, salary earned for first 10 hours in the
week  =  $ 8  10
=  $ 80
Let work hours for the rest of the week = x
Money earned for the rest of the week = $ 10  x
= $ 10x
Total earnings of the week = $ 80 + 10 x
Percentage of his earnings he saved every week = 90 %
i.e. 90% of total earnings = money saved
Since Ken saved atleast $ 270 in this week
90% of (80 + 10x) ≥ 270
(80 + 10x ) ≥ 270
(80 + 10x ) ≥ 270
9 (80 + 10x)  > 2700
720 + 90x ≥ 2700
90x ≥ 2700 - 720
90x > 1980
x  >  22
So, no of hours he should at least work is 22 hours
Hence, option C is correct