let t be 10% solution and f be 4% solution...
(0.1t+0.04f)/500=0.06 and t+f=500 so f=500-t making our equation:
(0.1t+0.04(500-t))=30
0.1t+20-.04t=30
0.06t=10
t=166 2/3 ml and since f=500-t, f=333 1/3 ml
So they obviously rounded to nearest ml...
167ml of 10% and 333ml of 4%