[dongping]

One can resort to traditional vertical multiplication (which requires patience), or do

897*394 = (900-3) * (400-6) = 900*400 - 6*900 - 400*3 + 3*6 = 360,000 - (5,400 + 1,200) + 18 = 360,018 - 6,600 = 353,418

[6510]

   8*3=24 and 800*300 =240000
   8*9=72 and 800* 90 = 72000
   8*4=32 and 800*  4 =  3200
   9*3=27 and  90*300 = 27000
   9*9=81 and  90* 90 =  8100
   9*4=36 and  90*  4 =   360
   7*3=21 and   7*300 =  2100
   7*9=63 and   7* 90 =   630
   7*4=28 and   7*  4 =    28
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                       353418