Stumped by wording of Q

kanha81 wrote:A certain school teaches 4th, 5th, & 6th graders only, with 150 students in each grade. During one day of a blizzard, 10% of 4th grade, 1/6 of 5th grade and 60 students of 6th grade did not attend the school. The student attendance on that day is approx. what percent less than full attendance?

1) 11%
2) 16%
3) 22%
4) 31%
5) 33%

I know how to solve the question all the way till the end and tht's where I mess up.

First calculate the number of missing students.
4th grade: 10% of 150 = 15
5th grade: 1/6 of 150 = 25
6th grade: 60
Total missing = 100
Attendance on day of blizzard = 450 - 100 = 350
Full attendance = 3x150 = 450

The question is asking us to determine the decrease in attendance.
Decrease = (450-350)/450 = 100/450 = 2/9
For the GMAT, you should memorize the percent approximations for all base fractions (i.e., 1/2 = 50%, 1/3 = 33.33% . . . . 1/9 = 11.11%)

So, if 1/9 = 11%, then 2/9 = 22% (and 7/9 = 77% etc)
The answer is C