25's+strategy

=The 25's strategy is based on the fact that 4 x 25 = 100.=

Example:
===17 x 25 You need to make 17 groups of 25 in your head. Instead think how many groups of four can I make out of 17, or what times four would get me close to 17? 17 divided by 4 is 4 remainder 1, or 4 x 4 = 16 one less than 17. For every group of four you make out of 17 you count 100 because 4 x 25 = 100. Therefore four groups of four 25's is 400. What about the remainder? The remainder will always be either 1, 2, or 3 in this case it is 1, so you simply add on 25, so the total is 400 + 25 = 425.===