I can't figure this out

A chemist mixed some 15% solution with some 6% solution to make 300 ml of a 9% soulution. How many ML of the 15% were used?


Please help me

You can set a formula like this:

X * 15% + Y * 6% = 300 * 9% and X + Y = 300

X says how much of 15% solution is used, and Y says how much of 6% solution was used. You want to know how much is X.

So you have a system of equations. You do like this:

Y = 300 - X that's from second formula
put that into first and you get
X*15% + (300 - X)*6% = 300*9%
x*15% - x*6% = 300*9% - 300*6%
multiply whole formula by 100 and get
15x - 6x = 300*9 - 300*6
9x = 300*3 = 900
x = 100

So, 100ml of 15% solution was used.