You have 1/2 equal to x/250. I'm making x be the placeholder for the number that goes in the blank slot.
We have this equation: 1/2 = x/250
Let's cross multiply to get 1*250 = 2*x which turns into 250 = 2x and that becomes 2x = 250
The last thing to do is divide both sides by 2. The division is to undo the multiplication done to x
2x = 250
2x/2 = 250/2
x = 125
So 125 goes in the blank slot meaning that 1/2 = 125/250 is a true equation. If you reduce 125/250, then you'll get 1/2 (divide both parts by 125). Or you can multiply top and bottom of 1/2 by 125 and you'll get 125/250