In an exhibition, some paintings were kept for sale. On the first day, 1 painting plus 1/7 th of the
remaining paintings were sold. On the second day, 2 paintings plus 1/7 th of the remaining paintings
were sold. A similar pattern continued till the kth day, when 'k' paintings were sold and no painting
was left after that. If the exhibition ran for exactly k days (k > 1), then what is the minimum number
of paintings sold during the exhibition?

Please explain the solution as well !!!

1) 36
2) 42
3) 99
4) 100
5) 81