How do you add fractions, with decimals in them. CONFUSED big time.

Ok here is Q#1

A tank can be filled by one of two pipes in 12.00 h and by the other of the two in 1.00 h. The same tank can be emptied by a drain pipe in 10.5h. If all three pipes are open, how long will it take to fill the tank.

Answer in time?

I know it's 1/12 + 1/1 - 1/10.5 = Need to find the time it takes to fill the tank.

I have no clue on subtracing a fraction with a decimal if you have to have the LCD to clear the fractions???

Please help thanks - PLEASE EXPLAIN how it's calculated.

Joanne