Mental calculation technique


Example Mental calculation Reason
398*4 1600-8=1592
398*4=(400-2)*4
=400*4-2*4
  =1600-8
240*5 120*10
divide by 2 and multiply by 10
240*5=240*10/2
=(240/2)*10
  =120*10
320/5 640/10
multiply by 2 and divide by 10
320/5=320/(10/2)
=(320*2)/10
  =640/10
760*200 calculate 76*2 and multiply by 1000
760*200=76*10*2*100
=76*2*1000
12*25 3*100
divide by 4 and multiply by 100
12*25=12*(100/4)
=(12/4)*100
  =3*100
13*99 1300-13
13*99=13*(100-1)
=1300-13


Japanese sites
Photograph| Mathematical Formulas| Kodawari House| Pinpoint StreetView|