Answer :
The easiest way to do a problem like this is to think about the fact that you have 100% and you want to add another 85%.
This is 185% of the quantity that you have.
Multiply 250 by 185% or 1.85.
250 * 1.85 = 462.5 ml
This is 185% of the quantity that you have.
Multiply 250 by 185% or 1.85.
250 * 1.85 = 462.5 ml