Answer:
it is likely that joanna was about 30 inches tall when she was born.
thats the answer
Step-by-step explanation:
Well after reading this 100 times I haven't found a 100% sure answer but I think its A
hope its right and it helps :D

We have that

, which means

reduces to

when

, or reduces to

when

.
So we can expand the summation as

Notice that the 10 contributes a total of 30 from the first sum, and -30 from the second sum, so those terms cancel, leaving us with
Customer 120, Customer Customer 120, and four customers