Solve 1/(8!) + 1/(9!) = x/(10!).
1 / 8! + 1 / 9*8! = x / (10*9* 8!)
9/(9*8!) + 1 /(9*8!)
10 / (9 *8!) = x / (10*9*8!)
100/ (10*9*8!) = x / (10*9*8!)
x = 100