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# Answer

12×12= (10 + 2) x (10 + 2)= (10x10) + (10x2) + (10x2) + (2x2)= 100 + 20 + 20 + 4 = **144**

20×20= (2 x 10) x (2 x 10)= (2x2) x (10x10) = 4 x 100 = **400**

12×13= (10 + 2) x (10 + 3)= (10x10) + (10x2) + (10x3) + (2x3)= 100 + 20 + 30 + 6 = **156**

8×19= 8 x (10 + 9)= 80 + 72 = **152**

40×40= (4 x 10) x (4 x 10)= (4x4) x (10x10) = 16 x 100 = **1600**