Frank can build a fence in twice the time it would take Sandy. Working together, they can build it in 7 hours. How long will it take each of them to do it alone?
Answer
If Frank and Sandy can build the fence in 7 hours, they must be building
1
7
of the fence every hour.
Now, let the amount of time it takes Sandy be
x
hours so that Frank takes
2
x
hours. Sandy can build
1
x
of the fence every hour and Frank can build
1
2
x
of the fence every hour.
We now have the following equation to solve.
1
x
+
1
2
x
=
1
7
2
+
1
2
x
=
1
7
21
=
2
x
x
=
21
2
=
10.50
Thus, Sandy takes
10.50
hours and Frank takes
21
hours.