Explanation:
remember, 1% = 1/100 = 0.01
4% = 0.04
6% = 0.06
so, we have to find the solution for
90×0.01 + x×0.06 = (90+x)×0.04
with x being the amount needed of the 6% solution.
90×0.01 + x×0.06 = 90×0.04 + x×0.04
90×0.01 + x×0.02 = 90×0.04
x×0.02 = 90×0.03
x = 90×0.03/0.02 = 135 ml
so, we need to add 135 ml of the 6% solution to get in total a 4% solution (90+135 = 225 ml of that).