to get the remainder zero we have to subtract 3 repeatedly from 15 and it's subsequent remainders. The logic is plain and simple, such fundamental knowledge doesn't make the student stereotyped and he can innovate and conclude the real life problems in a better manner. The less the students minimize their dependency on calculators, the better equipped they find themselves with faster calculations.
A quick way to estimate the difference between numbers is to round each number and then subtract the rounded numbers. This probably won't be the exact answer but it may be close enough for some purposes. An estimate can sometimes be improved. If the sum of 645-450 were estimated, we would round 645 to 600 and 450 to 500. The estimate would be 600-500 or 100. One number was rounded down and the other was rounded up. The number 645 was rounded down by 45 and 450 was rounded up by 50. Adding 45+50 gives 95, which rounds to 100. Therefore, a better estimate would be 200. The actual difference is 195.
Moreover, the students should make it a habit to calculate their bills, subtract the discounted price and to find out the remaining balance on their own. This will sharpen their skills. Nothing can be more blissful than Self- automation.