Explanation:

When four numbers are proportional, it means that they are in the same ratio. This can be expressed mathematically as:

1/2 : 1/3 : 1/5 : 1/x

To find the value of x, we need to determine the common ratio of the proportion. This can be done by multiplying the first two terms and the last two terms and setting them equal to each other:

(1/2) * (1/x) = (1/3) * (1/5)

Simplifying this equation, we get:

1/2x = 1/15

Multiplying both sides by 15x, we get:

15 = 2x

Dividing both sides by 2, we get:

x = 7.5

Therefore, the value of x is 7.5.

Summary:

- Four numbers are proportional if they are in the same ratio.
- To find the value of x, we need to determine the common ratio of the proportion.
- Multiply the first two terms and the last two terms and set them equal to each other to find the common ratio.
- Simplify the equation and solve for x.