If a varies directly as b, and a = 28 when b = 7, find b when a = 5. 4/5

a=y

b=x

revise:

y varies directly w/ x and y=28 when x=7, find x when y=5

y=kx

k=y/x

k=28/7

k=4

y=4x

5=4x

4x/4=5/4

x=5/4 or x=1.25

a = 4b and b = 5 / 4

Step-by-step explanation:

From given statement , we observe that

a ∝ b

a = kb        eq(1)    

where k is a constant.

given that

a = 28 when b = 7

28 = k(7)

k = 28 / 7

k = 4  

Putting  above value  in eq(1), we have

a = 4b

Now , we have to find value of b when a = 5.

Putting in above equation, we have

5 = 4b

b = 5 / 4 which is the answer.

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