If x/0.5 + y/0.2 = 18, where x and y are positive integers, then what is the value of x ?
explain how to do it

An equation without exponents and two variables, is typically a straight line. All the points on the line with integer coordinates are solutions of the equation. Since x and y have to be positive as well, there aren't that many solutions.

Let's see where the line crosses the x-axis, it is where y=0:

x/0.5 + 0 = 18, so x=9 at the intercept. y=0 there, so this is a point on the line, but not a solution to the question (y was supposed to be positive).

Possible values for x are thus limited to 1,2,3,4,5,6 and 7. You can try them all (ie., solve the equation with them) and see for which x values the y is also positive and integer.

You will find that x=4, y=2 is the only pair that satisfies these conditions.