While there are some ready rules that can be used to speed up calculations, all such questions can be solved by following mass balance - whatever you put in the solution stays there - and assuming the final volume is the sum of volumes. That produces two equations in two unknows, write them, solve, problem done.

Technically the answer won't be correct, as volumes are not additive (masses are) - the final volume is almost never exactly sum of volumes. But as long as the solutions are reasonably diluted the error is negligible.