1) 6 3/4 - 2,9 = 6 3/4 - 2 9/10 = 6 15/20 - 2 18/20 = 5 35/20 - 2 18/20 = 3 17/20
2) 3 17/20 * 4 = 77/20 * 4 = 77/5 = 15,4
3) 7 5/8 - 3,25 = 7 5/8 - 3 1/4 = 7 5/8 - 3 2/8 = 4 3/8
4) 4 3/8 : 7/8 = 35/8 * 8/7 = 5
5) 15,4 + 5 = 20,4
3мин=1мин*3=60с*3=180с; 2ч=1ч*2=60мин*2=3600с*2=7200с; 1ч2мин=62мин=3720с; 1ч24мин=84мин=5040с
180с=3мин; 240с=4мин; 420с=7мин; 6660с=111мин(=1ч51мин)
82с=1мин12с; 200с=3мин20с; 422с=7мин2с; 6666с=111мин6с(=1ч51мин6с)
9-2х=4х-15
9х-2х=-(4х-15)
х1=3
х2=4
1/6-1/5= 5/30-6/30= - 1/30