x = the amount the hamsters ate on the first day
\(x + x+20 + x+40 + x+60+x+80+x+100+x+120 = 1260 \\ 7x+420=1260 \\ (7x+420)-420=(1260)-420 \\ 7x = 840 \\ (7x)/7 = (840)/7 \\ x=120\)
The hamsters were fed 120g of food on the first day. Hope this helps!