Найдите сумму членов от20-го по 25-й арифметический прогрессии an, если а1=2,d=2.

i'm not sure that's right, but folowing my minds:

as k+k=t and t+t=k it means that t=k. so it doesn't matter what number to substitute for letters, and c=0. hope, i helped you=)

пусть осталось х

1,4х-израсходовано

х+1,4х=180

2,4х=180

х=75кг