Total number of students in the class = 36
Number of students wearing blue shirts = (1/6) * 36
= 36/6
= 6
Number of students wearing white shirts = (2/3) * 36
= 2 * 12
= 24
Then
Number of students not wearing blue or white shirts in the class = 36 - (6 + 24)
= 36 - 30
= 6
So 6 students in the class are wearing a shirt other than a blue or a white shirt. I hope the procedure is clear enough for you to understand.
The critical points are at x = 1 and x = 4 giving you the intervals (-inf, 1), (1, 4) and (4, inf).
By substituting x values in these 3 intervals, you can see that f'(x) is positive in the first and third intervals and negative in the second interval.
This means that f(x) is increasing in the first and third intervals and decreasing in the second interval.
The answer is D.
Y = 12.1% x 13 + 351
y= 0.012 x 13 + 351
y= 1.573 + 351
y= 352.573 or $352.6
