Let C denote the 4% solution and D denote the final 18% solution.
We have 7 oz of 34% solution.
Using the method of alligation.
16= 18 - 34
14= 4 - 18
E.g. There are 14 parts of C for every 16 parts of D
then there are 7 parts of z, since (14/16)=7/z
x= 8.0= 7/(14/16) oz of 4% solution to add to your 7 oz of 34% solution.
y= 15.0= (7+8.0) oz of the final 18% solution.
check
0.04 * 8.0 + 0.34 * 7 = 0.18*(7+8.0)
0.32 + 2.38 = 0.18*(15.0)
2.7 = 2.7
ok
34 14 7 oz of 34% solution
18 15 oz of 18% solution
4 16 16/14*7=8 oz of 4% solution
30