Let A=amount of 10% acid and B=amount of 4% acid. So A+B=150cc. B=150-A.
Also, 0.1A+0.04B=0.06×150=9cc, which we can write as:
0.1A+6-0.04A=9, 0.06A=3, so A=3/0.06=300/6=50cc making B=150-50=100cc.
Therefore we need 50cc of the 10% solution and 100cc of the 4% solution. (There is 5cc of acid in the 10% solution and 4cc in the 4% solution, making 9cc altogether, which is 6% of the mixture.)