Let 'x' be the amount @ 10% saline solution

'y' be the amount @ 15% saline solution

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-since the total mixture amounts to 100mL then

x + y = 100 ----equation 1

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-the resulting solution is 12% concentrated

0.1x + 0.15y = 100(0.12)

0.1x + 0.15y = 12 ------equation 2

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From equation 1

x + y = 100

x = 100 - y -------equation 1'

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Substitute equation 1' to equation 2

0.1x + 0.15y = 12

0.1(100-y) + 0.15y = 12

10 - 0.1y + 0.15y = 12

-0.1y + 0.15y = 12 - 10

0.05y = 2

__y = 40mL__

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Substitute y=40 to equation 1'

x = 100 - y

x = 100 - 40

__x = 60mL__

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Therefore he used **40mL of 15%** saline solution and **60mL of 10%** saline solution.