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Question

A container has 3 kg of phosphorus which is 81% strong. It is made from two mixes of 75% and 90% strength. How much of each mix is there?

Answer 1

90x + 75(3-x) = 81(3)

90x -75x = 81(3) - 75(3)

15x = (81-75)(3) = 6(3) = 18

x = 18/15 = 6/5 = 1.2 kg of 90%

3-x = 3-1.2 = 1.8 kg of 75%

81 is within 9 of 90

75 is within 6 of 81

9/6 = 3/2

1.8/1.2 = 3/2

75 is closer to 81, so you know you

need more of it, precisely 1.5 times more