Criss cross using negative values:
90 -1
50 -3
_______ X
90X50 = 4500
(-1 X 50) + (-3 X 90) = -50 - 270 = -320
-1 X -3 = 3
__________________ +
4500
-320 (=4180)
-+3 (=4183)
Anchor method using 50 as anchor:
1st step. We add 3 to 47 to get 50.
Therefore we subtract the same 3 from 89 to get 86.
Now multiply them:
50 X 86 = 4300
Now subtract the difference from the base numbers (we changed 47 into 50 and 89 into 86. So difference is 3) and multiply by the distance of the anchor and the biggest number:
-3 X (89 - 50) = -3 X 39 = -117
4300 -117 = 4183
Btw this is based on: (a+c)(a+d) = a(a+c+d) + cd
Criss cross (using negative values):
90 -3
40 -1
__________ X
3600
-90-120 = -210 (total is 3390)
–1X-3 = 3 (total is 3393)
Anchor using 40:
40 X 86 = 3200 + 240 = 3440
-1 X (87-40) = -1 X (47) = -47
3440 -47 = 3393
You privately asked me:
Anchor method seems quick:
50 X 77 = 3850
3 x 24 = 72
3850 + 72 = 3922
Either do:
70 -3
90 -3
Or anchor:
60X94 + 7X27
Either do:
70 -3
80 -1
Or anchor:
60X86 + 7X19