Question

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

A 4/5
B 5/4
C 39.2

Answer 1

Answer: Option B.

$b=\frac{5}{4}$

Step-by-step explanation:

If a varies directly with b then when a increases b it also increases by a factor k.

Then we can write that

$a = kb$

We know that when a = 28 then b = 7. So

$28 = k * 7$

$k=\frac{28}{7}$

$k=4$

Therefore the equation of proportionality is:

$a=4b$

So when $a=5$  then:

$5=4b\\\\b=\frac{5}{4}$

Answer 2

ANSWER

The correct choice is B.

EXPLANATION

If 'a' varies directly as 'b' then we write the equation of variation,

$a = kb$

Where k is the constant of variation.

We have that a = 28 when b = 7.

This implies that,

$28= 7k$

$k = 4$

The equation now becomes

$a = 4b$

When we a=5, we have

$5 = 4b$

$b = \frac{5}{4}$

The correct answer is B