Question

A coin is weighted so that the probability of obtaining a head in a single toss is .4. If the coin is tossed 25 times, what is the probability of obtaining:
a) Fewer than 10 heads?
b) Between 10 and 12 heads, inclusive?
c) More than 15 heads?

This is what I did:
I found the mean : 25 x .4= 10
Then the standard deviation: the square root of 25x.4x.6=2.45

for part a : 10-10/2.45= 0 =0.5000
for b: p(x<10) = 10-10/2.45=0=0.5000
p(x<12) = 12-10/2.45=0.82=0.2939
for c: p(x>15) = 15-10/2.45=2.04--->-2.04=0.0207
but my textbook's answer key says I'm wrong.