By what percent will the product of two numbers decrease if one of them is decreased by 25% and the other is decreased by 50%?

Well I don't know !
Let's get together and figure it out:

The product of ' A ' and ' B ' is   (A B) .

If you decrease ' A ' by 25%, then ' A ' becomes ' 0.75 A ' .

If you decrease ' B ' by 50%, then ' B ' becomes ' 0.5 B ' .

Now their product is      (0.75 A) x (0.5 B) .

In multiplication, the order of the factors doesn't matter.
So we car re-arrange the new product as

                                       (0.75 x 0.5) x (A B) .

But (A B) is the original product, before we started
messing around with ' A ' and ' B '.

So the original product has become

                                       (0.75 x 0.5) = 0.375 of itself .

The original product has been reduced by

                             (1 - 0.375) = 0.625  of itself.

C. i believe it is (5×(-20))+(100×(0.7))