Question

Three friends bought 4, 6, and 8 books. The table shows the cost of the books.

Answer 1

I'd say its 15 bc that goes into all of them 

Answer 2

Answer:

Option (B) is correct .

The constant of proportionality is 7.5 .

Reason

Let us assume that the number of books be x.

Let us assume that the cost of books be y .

Than

$y \propto x$

y = kx

Where k is constant of proportionality .

As given

Number of Books 4 6 8

Cost ($) 30 45 60

Thus

when x = 4 , y = 30

Put in the equation y = kx .

$30 = 4k$

$k = \frac{30}{4}$

k = 7.5

when x = 6 , y = 45

Put in the equation y = kx .

$45 = 6k$

$k = \frac{45}{6}$

k = 7.5

when x = 8 , y = 60

Put in the equation y = kx .

$60 = 8k$

$k = \frac{60}{8}$

k = 7.5

Therefore the constant of proportionality is 7.5  i.e option (B) is correct .