You can 
put this solution on YOUR website! 
A fair coin is tossed 7 times. What is the probability of obtaining exactly 4 heads?
Answer = 
or you can get it on a TI-83 or 84 by
binompdf(7,1/2,4) 
ENTER
You get binompdf( by pressing 2ND VARS scroll down to binompdf(  
then press ENTER
You'll get the same answer .2723275.
Edwin