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.