Задание 4 :

а) 1/2 * x + 1/3 * x = 6

умножим все на 6

3x + 2x = 36 => 5x = 36 => x = 36/5

б) 1/5 * x + 1/3 * x = 15

умножим все на 15

3x + 5x = 225 => 8x = 225 => x = 225/8

Задание 5 :

мальчиков : 24 * 2/3 = 48/3 = 16

девочек 24 * 1/3 = 24/3 = 8

Задание 6 :

5 - 8 * (1 1/6 - 2/3) / 1 1/5 = 5 - 8 * (7/6 - 2/3) / (6/5) = 5 - 8 * (7/6 - 4/6) / (6/5) = 5 - 8 * 3/6 / (6/5) = 5 - 8 * 1/2 * 5/6 = 5 - 8/2 * 5/6 = 5 - 40/12 = 60/12 - 40/12 = 20/12 = 5/3