SOLUTION: A pharmacist wants to make 18 ml of a 6% hydrogen peroxide solution. How many mls of 5% hydrogen peroxide must he mix with 7% hydrogen peroxide to make the desired solution?

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Let = ml of 5% solution needed
Let = ml of 7% solution needed
= ml of hydrogen peroxide in 5% solution
= ml of hydrogen peroxide in 7% solution
----------------------------------------
given:
(1)
(2)
--------------------------
(2)
(2)
(2)
Multiply both sides of (1) by and
subtract (1) from (2)
(2)
(1)


and, since
(1)
(1)
(1)
9 ml of 5% solution is needed
9 ml of 7% solution is needed
check:
(2)
(2)
(2)
(2)
(2)
(2)
OK