If p + q=16 and
p+10=20
what is the possible value of q?

P + 10 = 20   Subtract 10 from both sides
       P = 10  

Substitute P = 10 into P + Q = 16

 P + Q = 16   Plug in 10 for P
10 + Q = 16   Subtract 10 from both sides
        Q = 6

The answer is 6 because p+10=20 you subtract 10 from both sides and get p=10.
Then you subsitute 10 in for p in the equatin P=Q=16 and 10 +Q=16 because the only answer is 6

What mathematical statement is missing ? ? the answer is b