Given:
Bernoulli trials,
(population) success rate = p = 0.7 (constant throughout trials)
Assume independence of the 50 batteries
use binomial distribution with parameters (n,p)=(50,0.7)
P(X\in {25,26,27,28,29,30})
=0.001436+0.003223+0.006684+0.012811+0.022677
=0.037039