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It works because they are the same numbers. 20+58=78. 7+8 is 2+0+5=7+8. It's just that when you do 20+58 you are taking more time than when you do 2+0+5+8. Do you understand? In a shorter way, they are the same numbers just in a dfferent order. If you take out the 8 in 2+0+5+8 you get 2+0+5 which is 7. Then you add 7 and 8 which is what you end up with in 20+58=78 7+8. Becuase you are adding you will get the same answer.
on June 21, 2012