Let x be the quantity of the 1st solution (at 10%) and y, the 2nd (at 4%)
At the end we will have a TOTAL = x + y = 500 ml (mind you this is a volume)
But we need 10% of x and 4% of y to have a TOTAL mixture of 6%
Or 0.1x + 0.04 y = 0.06 (of 500)
Solve :
x + y = 500
0.1 x + 0.04 y =30
You will find:
x= 500/3 = 166.66 ml
y = 1000/3 = 333.33 ml
Apply this methodology for all other items to get the right answer