The answer is twelve sevenths (12/7)
x - the job
t - time to do the job x
Robin can mow a lawn in 3 hours: x/3 * t
Brady can mow the same lawn in 4 hours: x/4 * t
If they work together, the total work will
be the sum of their speeds:
x = x/3 * t + x/4 * t = (x/3 + x/4)*t = (4x/12 + 3x/12) * t = 7x/12 * t
x = 7x/12 * t
Divide both sides by x:
x/x = 7x/12 * t/x
1 = 7/12 t
t = 1 : 7/12
t = 12/7
