Question

A cellphone store has 120 cell phones in stock. Of these, 45 have keyboards. The manager of the store wants to add more cell phones with keyboards so that 40% of the stock has keyboards.

Answer 1

5

Explanation:

Let x be the number of cell phones with keyboards needed.

40% =
`(40)/(100)`

`(x+45)/(x+120)` =
`(40)/(100)`

x+45 = 40 -----1

x+120 = 100 -----2

You would want to make the total of 1 the same as 2.

1×
`(100)/(40)`: 2.5+112.5 = 100 -----3

Now 2 and 3 are equal.

x+120 = 2.5x+112.5

Isolate the x and numbers.

120-112.5 = 2.5x-x

Simplify.

1.5x = 7.5

x =
`(1)/(1.5)` × 7.5

x = 5

The manager of the store needs to add 5 more cell phones with keyboards.