121 balls are placed in group of 7and 5 in basket.how many baskets 0f 7 were there?

Answer 1

An obvious solution would be
7*3+5*20=121
Consequently, change 5*7 for 7*5 gives
7*(3+5)+5*(20-7)=7*8+5*13=121
Similarly,
7*(8+5)+5*(13-7)=7*13+5*6=121
We cannot go further because number of baskets for groups of 5 will be negative.

So possible values for baskets of 7 are:
3,8,13