Alab technician wants to make 50ml of a 20% alcohol solution. she has solutions that are 30% and 15% alcohol. how much of each must she use?

Make 2 equations:-
let x be volume of 15% and y the volume of the 30% solution

x + y = 50
0.15x + 0.30y =  50*0.2 = 10

from first equation y = 50 - x, so:-

0.15x + 0.30(50 - x) = 10

0.15x + 15 - 0.30x  = 10

5 =  0.15x

x = 5 / 0.15 = 33.33 mls

and y = 50 - 33.33 = 16.67 mls

she needs 33.33 mls of the 15% solution ans 16.67 of the 30%.