Im stuck on an equation in my math class help is needed if possible

Question

To Build the garden of your dreams you need 18 meters cubes of soil containing 50% silt. You have two types of soil you can combine to achieve this: soil with 10% silt and pure silt. How much of each soil should you use?

Answer 1

Let x = the amount of 10% silt

18-x = the amount of pure silt

then,

(10x + (18-x)100)/(x + 18 -x) = 50

10x + 1800 - 100x = 900

-90x = -900

x = 10 The amount of 10% silt

so. 18-x = 8. The amount of pure silt