Question 1009788
Call the amount of 20% solution you are adding, x.  The set up is like this
.05(2.5) + .20x = .12(2.5 + x)
.125 + .2x = .3 + .12x
.08x = .175
x = .175/.08 = 2 3/16 kg or 2.1875 kg of 20% solution