1、
[5/12 - (1/3 - 1/7)] ÷ (19/21)
= (5/12 - 4/21) × 21/19
= 105/228 - 4/19
= 105/228 - 48/228
= 57/228
= 1/4
2、
12 ÷ (4/7) - 20×5％
= 21 - 1
= 20
3、
12.1 - (1/15 + 1/21) × 105
= 12.1 - (7/105 + 5/105) × 105
= 12.1 - 12/105 × 105
= 12.1 - 12
= 0.1
4、
(870 + 870 + 87) ÷ 87
= 10 + 10 + 1
= 21