Question 618635
y varies directly with x. y =15 when x=4. what is the value of y when x=5
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When you see "y varies directly with x" think:
y = kx (where k is a constant)
find this constant by plugging in "y =15 when x=4" from the problem:
15 = k(4)
15/4 = k
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Now, our equation is:
y = (15/4)X
We can now answer:
what is the value of y when x=5
y = (15/4)X
y = (15/4)5
y = 75/4
y = 18.75
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y varies inversely with x y=8 when x=8. what is the value of y when x=4 
When you see "y varies inversely with x" think:
y = k/x (where k is a constant)
find this constant by plugging in "y=8 when x=8" from the problem:
8 = k/8
8/8 = k
1 = k
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Now our equation is:
y = 1/x
We can now answer:
what is the value of y when x=4
y = 1/x
y = 1/4
y = .25