Which is a correct way of evaluating the expression 30b+12b for b=5?

The answer is 210 because you multiply 42 x 5 = 210

for ex: the reciprocal of 3/4 is 4/3. it is the opposite.

reciprocal definition:

a mathematical expression or function so related to another that their product is one; the quantity obtained by dividing the number one by a given quantity.