You have two containers of acid solution. One is 30% acid. The other is a 50% acid solution. How much of each should be added to get a 45% acid solution?

45 is 3/4 of the way from 30 to 50, so 3/4 of the mix will be 50%.

Or, more traditionally,

30x + 50y = 45(x+y)
30x + 50y = 45x + 45y
5y = 15x
y = 3x

So, there is 3 times as much 50% as 30%.