Tính nhanh: 3 + 6 + 7 + 4 =  19 + 17 + 13 + 11 = 99 + 97 + 98 + 96 =

3 + 6 + 7 + 4 = (3 + 7) + (6 + 4) = 10 + 10 = 20
19 + 17 + 13 + 11 = (19 + 11) + (17 + 13) = 30 + 30 = 60
99 + 97 + 98 + 96 = 100 – 1 + 100 – 2 + 100 – 3 + 100 – 4
= (100 + 100 + 100 + 100) – (1 + 2 + 3 + 4)
= 400 – 10
= 390