First we see whether the normal approximation is valid by 
calculating np and n(1-p) to see whether they are both at 
least 5. np = (50)(0.4) = 20 and n(1-p) = (50)(1-0.4) =
(50)(0.6) = 30.  So the approximation is valid.






The word "inclusive" follows the words "between 15 and 
25 times".  Therefore, we get the area under the normal curve 
from 0.5 less than the smaller value 15, that is, from x=14.5 
to 0.5 greater than the greater value 25, that is, to x=25.5.
We can do this either with technology or with a normal table.

With a TI-84

normalcdf(14.5,25.5,50*0.4,√(50*.4*0.6))

We get 0.8876487995

[Personally, I doubt that is any more accurate than 0.8 or 0.9]

Edwin