45% of 900 kg is 405 kilograms
40 is the answer for the equation 20+20=40
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 :)