1) 980-420=560(руб.)-на тетради дочери.
2)560:40=14(шт.)
Ответ: 14 тетрадей купила мама.
С из n по k равно n факториал делить на произведение k факториал и (n-k) факториал.
1,6x>-3; x<-3:1,6; x<-1.875
-4x<-2,7; x>-2.7:(-4); x<0,675
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У+134 ×4=600
y+ 536 = 600
y = 600-536
y = 64
проверка: 64 + 134 * 4 = 600
64+ 536 = 600
600 = 600