Within how many standard deviations of the mean do all of the values fall 11 14 10 8 12 13 12 18 9 12 16 15 11 9 10

The mean is 12 and std dev 2.8
The max is 18 and min is 8
One std dev would be:
If you add 2.8 + 12 = 14.8
If you subtract 12-2.8 = 9.2
Two std dev would be:
14.8 + 2.8 = 17.6
9.2-2.8 = 6.4
All of the data falls within 2 std deviations to the left and 3 std deviations to the right of the mean.

The answer is $9.90 with rounding to the nearest tenth