You want to subtract 620-89.7. If you want to use the standard algorithm you have 620.0-89.7. You "borrow" from the 2 in the tens place, then "borrow" from the ones place. To subtract 10-3 tenths. Then you subtract the ones place which is 9-9=0 ones. Now you subtract 62-8=53 tens. So you have 530.3 as the difference.
When construction workers need measurements that are fractions they need to be able to multiply/add/subtract/divide all of those different fractions and mixed numbers.
