Acoin is tossed seven times and the sequence of heads and tails is observed. what is the number of different outcomes having exactly three heads?

You could make a tree diagram
                  H                          T                               ← 1st toss

           H           T               H          T                       ← 2nd toss

     H       T    H      T       H     T    H    T                   ← 3rd toss
etc ,,, then count the combination of favorable outcomes

Using a calculator:
Total number of outcomes is  = 128
Probability of getting exactly 3 heads is found using the follow calculator feature: binompdf(7, 0.5, 3) 

ANSWER: Typed into the calculator as follows binompdf(7,0.5,3)×128 = 35

33.3333333333 that should be the answer