63×8 =504 because you are using repeated addition r just regular multiplication
One fifth is the same as

and 15 is the same as

So just times across,

×

Resulting in(1 x 15= 15, and 5 x 1= 5),

If you need/want to reduce it, divide both numbers by 5. Leaving you with the simplified answer of

or 3. Either answer works. :)
If you have any questions, just comment them down below :)
F(x<40)= 0.39+(x*0.24) This formula will quantify what you have written out above.
What are you asking exactly