If one-eighth of one-fifth of a number is 15, then three-tenths of that number is:

Step 1: Let's denote the unknown number as "x". The problem states that one-eighth of one-fifth of this number is 15. We can write this as an equation: (1/8) * (1/5) * x = 15.

Step 2: Simplify the left side of the equation. (1/8) * (1/5) is equal to 1/40. So, our equation now reads: (1/40) * x = 15.

Step 3: To solve for x, we need to get rid of the fraction on the left side. We can do this by multiplying both sides of the equation by 40. This gives us: x = 15 * 40.

Step 4: Solve the equation. x = 600.

Step 5: Now that we know x is 600, we can find three-tenths of this number. This is done by multiplying 600 by 3/10, which gives us 180.

So, three-tenths of the number is 180.