-oh- you took one away from the four and you stick that one in front on the two but you're really taking a 100 from the 500, making it 400 and then adding that 100 to the 20 here and making it 120 but your writing here is a 12 because it is 12 tens you're at the tens place so let me write it down this is the ones place this is tens place and this is the hundreds place so now that our numbers on top in the tens place is bigger than the numbers on the bottom we can subtract so we get 120-70 that is 50 or 12 minus 7 is 5 5 is in the tens place so it's really representing 50