Question

We have two varieties of concrete that we can mix together. One is 5% water and 95% concrete mix, and the other is 20% water and 80% concrete mix. We want to end up with 30 pounds of a result that is 10% water and 90% concrete mix. Determine how many pounds of each variety we need.

Answer 1

Answers:

• 20 pounds of the 95% concrete mix
• 10 pounds of the 80% concrete mix

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Explanation:

x = amount of the 95% concrete mix

y = amount of the 80% concrete mix

Type A mix has 95% concrete. If we have x pounds of this, then 0.95x pounds is pure concrete. Type B mix has 80% concrete. If there are y pounds of this, then 0.80y pounds is pure concrete.

So far we have 0.95x+0.80y pounds of pure concrete from the two mixes

Divide this over 30 to get (0.95x+0.80y)/30

Setting this equal to 0.90 means we want the final mix to be 90% concrete.

Since we want 30 pounds total, we can say x+y = 30 which solves to y = 30-x. We'll use this later

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(0.95x+0.80y)/30 = 0.90

0.95x+0.80y = 30*0.90 .... multiply both sides by 30

0.95x+0.80y = 27

0.95x+0.80(30-x) = 27 ..... plug in y = 30-x

0.95x+0.80*30-0.80*x = 27

0.95x+24-0.80x = 27

0.15x+24 = 27

0.15x = 27-24 ..... subtracting 24 from both sides

0.15x = 3

x = 3/0.15 ........... dividing both sides by 0.15

x = 20

y = 30-x

y = 30-20

y = 10