Question

Can anyone help with #4

Answer 1

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Define x and y
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Let x be the amount of the cream (which has 2.5% of hydrocortisone)
Let y be the amount of the water-washable base. (which has 0% of hydrocortisone)

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Construct the 2 equations:
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Both x and y must add up to 120g
⇒ x + y = 120

Both x and y must add up to only 1% of hydrocortisone.
⇒0.025x + 0.0y = 0.01 x 120
⇒0.025x = 1.2

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Solve x and y:
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x + y = 120    ----------------- (equation 1)
0.025x = 1.2  ----------------- (equation 2)

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From equation 2:
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0.025x = 1.2
x = 1.2 ÷ 0.025
x = 48 

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Substitute x = 48 into equation 1:
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x + y = 120  
48 + y = 120
y = 120 - 48
y = 72

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Answer: 48g of 2.5% of the cream and 72g of the water washable base are needed to make 120g of the prescription. 
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