We can use the mixture shortcut to solve this within seconds.
Conc. A has 8% acid, Conc. B has 15% acid, the mixture of both of them has 12% acid.

We just have to take the differences between 12% (the mixture) and each of concentration A and B, and assign the quotient to the other concentration (look at the figure above), to get the RATIO of Volume.

So we get the ratio of volume for 8% and 15% concentration as 3 : 4

Given, the nurse uses 120 ml of the 8% solution
So the volume of 15% conc. = (120/3) * 4 = 160 (Answer)

Hope this helps!