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

number of books 4 6 8
cost ($) 30 45 60
what is the constant of proportionality?

a. 2

b.7.5

c.15

d.30

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 = kx

Where k is constant of proportionality .

As given

Number of Books468

Cost ($)304560

Thus

when x = 4 , y = 30

Put in the equation y = kx .

k = 7.5

when x = 6 , y = 45

Put in the equation y = kx .

k = 7.5

when x = 8 , y = 60

Put in the equation y = kx .

k = 7.5

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