We know that multiplication is repeated addition. For example, 5(5) = 5+5+5+5+5. It's 5 added five times.
It's no different here. That means we have x 1000 times. If we look at a simpler situation:
5x - 3x, we have (x + x + x + x + x) - (x + x + x)
That means we're getting rid of 3 x's, which means we have 2x. If we have different coefficients but the same variable, we can just subtract or add the coefficients but keep the variable. That means:
The answer is 10 pairs of shoes. The first ratio is 10:5. 10 socks per 5 pairs of shoes. You have 20 socks now instead of 10. You just double the ratio. 20 socks per 10 pairs of shoes. 20:10.