(100x - y)/(x^2 + 1)
I got the same anwer..
A takes x hours/deck
B takes 1/x hours/deck
in 1 hour, A completes 1/x decks
in 1 hour, B completes x decks
in 1 hour, A+B complete (1+x^2)/x decks
number of decks completed by A in y hours = y/x
number of decks left by the time B joined A = 100 - y/x = (100x-y)/x
Number of hours A and B worked together = ((100x-y)/x)/((1+x^2)/x)
The path is long, but self-surrender makes it short;
the way is difficult, but perfect trust makes it easy.