How much 45% acid solution should be mixed with a 30% acid solution to make 450 ml of a 40% acid solution?

We do as follows:

let x = amount of 45% solution
    y = amount of 30% solution

overall balance =>    x + y = 450
acid balance     =>    .45x + .30y = .40(450)

Solving simultaneously, we will have:

x = 300 mL
y = 150 mL

This is assuming that partial molar properties does not affect the volume greatly. Hope this answers the question. Have a nice day.