**Answer:**

One store sold 27, and the other sold 43.

**Step-by-step explanation:**

So, we can easily set this up in an equation. X represents the number of soccer balls in store one, and Y represents the number of soccer balls in store two. Since there's a total of 70 soccer balls...

x+y=70

And since we know how much money they sold for and made total...

15x+65y=2400

We'll grab that first equation and change it around a bit. We'll make it an equation where we're solving for y. Remember that when we switch a value from one side to the other, that value becomes negative. Since x is going to the other side, the x becomes negative.

y=-x+70

We can plug this equation into our second one.

15x+65(-x+70)=2400

We distribute the 65 to the values inside the parentheses.

15x-65x+4550=2400

We only want x on the left side, so we're going to subtract 4550 from both sides.

15x-65x=-2150

Then we can subtract 15x by 65x, resulting in another negative number.

-50x=-2150

Then, we just divide both sides by -50. A negative and a negative makes for a positive.

x=43

Now we know one store sold 43 soccer balls for $15 each. We can plug our new x into the first equation.

43+y=70

Then we subtract both sides by 43.

y=27

Now we know the other store sold 27 soccer balls for a whopping $65 each. Just to double check, we'll plug our numbers into the other equation, too.

15(43)+65(27)=2400

645+1755=2400

Yep! So, we end up with one store selling 43 and the other selling 27.