Question 999139
x = time it takes Bob to mow the lawn by himself (no one else helps)
y = time it takes Al to mow the lawn by himself (no one else helps)
z = time it takes Bob and Al to mow the lawn if they work together. This is assuming they work the most efficiently and don't get in each other's way


The values of x,y,z are times in hours


We're given "Bob takes 3 hours to mow a lawn. Al takes 4 hours" so


x = 3
y = 4
z = unknown but we can solve for it


1/x + 1/y = 1/z
1/3 + 1/4 = 1/z
4/12 + 3/12 = 1/z
7/12 = 1/z
12/7 = z/1
z/1 = 12/7
z = 12/7


If they work together, it takes exactly <font color="red">12/7 hours</font> which is approximately <font color="red">1.714 hours</font> (rounded to the nearest thousandth).