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Wiki User
65*39 = 65*(40-1) = 65*40 - 65*1 = 2600 - 65 = 2535
To calculate 65*40 in your head, you can try
6.5 is half of 13 ie 6.5*2 = 13
so 65*2 = 130
so 65*4 = 260
and so 65*40 = 2600