Together, bob and tom take 4 hours to get the yard work done. if bob works alone, it takes him 6 hours. how long would it take tom, working alone, to do all of the yard work?

10 hours
12 hours
8 hours
9 hours

The answer is 12 and let me tell you how can you get to that conclussion:
Let  X be the number of hrs Tom takes to do it
Add their rates of working
( 1 yard ) / ( 6 hrs ) + ( 1 yard ) / ( X hrs ) = ( 1 yard ) / (4 hrs ) and then what you do is
1/6 + 1/x = 1/4
Remember to multiply both sides by 12x
then = 2x + 12 = 3x