Let us say Ahmad has x marbles.

Then Sara has 4x.

When sara gives 18 to Ahmad then;

Sara has 4x-18

Ahmad has x+18

It says that 4x-18 = x+18

4x-18 = x+18

3x = 36

  x = 12

Ahmad had 12 marbles

Sara had 48 marbles

First you need to make an equation. As Ahmed has the least number, start with him.

Let x=Ahmed's marbles

`therefore`  4x=Sara's marbles

Sara gives away 18 marbles to Ahmed and then both sides of the equation are equal.

`therefore` 4x-18 = x+18

`therefore` 4x-x = 18+18

`therefore` 3x = 36

`therefore` x=36 `divide` 3

`therefore ` x=12

So Ahmed has 12 marbles to start with and, as Sara started off with four times as many, she has 12 x 4 which is 48.

Now they both have the same = 30