First we have to subtract 72 from 604. But we can do it into two steps. Then it makes much easier.
604-72=600+4-70-2=600 +2-70=530+2=532
And now we want to obtain 604 again. But before we calculate it, think what we already did. We subtract 72 from 604 and our result was 532. Now we want back to 604 again, so we have to add 72 to 532. In few steps to do it easier.
532+72= 530 + 2+ 70 + 2= 600 + 4=604
