The confidence interval is from 0.2191 to 0.2649, given by

.
The z-score we need is found by first subtracting the confidence level from 1:
1 - 0.95 = 0.05
Divide that by 2:
0.05/2 = 0.025
Subtract that from 1:
1 - 0.025 = 0.975
Look this up in the middle of a z-table (http://www.z-table.com) and we see that the z-score is 1.96.
Now our confidence interval is given by
Tan(-x)csc(-x)sec -1 (-x)cot(-x) i am not sure, I hope you get a good grade, this is what i got.. ',: (
All you would do is multiply .9 by 20
.9*20=18
90% of 20 is 18
100/40×20.
100/40=2•5
2•5×20=50
the answer is 50
(325-120)/2.5=2050/25=82
=>82 students attended the dance