The number of responders has a binomial distribution B(0.25,5000). You are

expected to use the normal approximation.

The mean of the binomial is m=0.25 * 5000 = 1250, and standard deviation

s=sqrt(5000*0.25*0.75) ~= 30.6.

1000 or fewer corresponts to less than a z-score of (1000.5 - m)/s, which

you look up in your normal table (the extra 0.5 is a continuity correction)

3000 of more corresponds to more than a z-score of (2999.5-m)/s.

RonL