дана пропорция, освновной свойство пропорции: произведение крайних членов равно произведению средних.
1)
а/5 = 6/15
а * 15 = 5 * 6
15а = 30
а = 30 : 15
а = 2
.................
2)
1/12 = 4/а
1*а = 12 * 4
а = 48
.................
3)
56/70 = 8/а
56 * а = 70 * 8
56а = 560
а = 560 : 56
а = 10
............
4)
а/60 = 6/5
а * 5 = 60 * 6
5а = 360
а = 360 : 5
а = 72
............................................
или
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