ДУМАЕМ Два неизвестных - V и R - пишем два уравнения. РЕШЕНИЕ 1) V + R = 19 1/2 - скорость по течению 2) V - R = 7 3/5 - скорость против течения Сложили два уравнения и получили 3) 3) 2*V = 19 1/2 + 7 3/5 = 27 1/10 Находим неизвестное V 4) V = 27 1/10 : 2 = 13 11/20 = 13.55 км/ч - скорость катера (не нужна) Вычитаем уравнения - 1) - 2) = 5) 5) 2*R = 19 1/2 - 7 3/5 = 11 9/10 Находим неизвестное R/ 6) R = 11 9/10 : 2 = 5 19/20 = 5.95 км/ч - скорость течения - ОТВЕТ СПРАВОЧНО. Можно догадаться, что вычислять скорость катера НЕ НАДО и уравнения 3) и 4) из решения УДАЛЯЕМ.
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сначала мы узнали сколько стоит 1 чашка ( 240:6 ) а затем умножили на 4 узнав сколько стоят 4 чашки