You can build a snow fort in 6 hours. If your sister helps you, it will only take 4 hours. How long would it take for your sister to build it by herself?

Question

All I really need is the formula for this problem. I've thought about it for 2 days and I can't seem to figure out where to start.

Answer 1

This problem trips up a lot of people, because you have to use the reciprocal of the information given.  If you can build a snow fort in 6 hours, that means for every hour, you build 1/6 of a fort.  If the two of you work together, that means you complete 1/6 + the reciprocal of your sisters time to complete it on her own, for every hour, to add up to the reciprocal of the total time.

 

You: 6 hours (1/6 per hour)

Your sister: x hours (1/x per hour)

Total time: 4 hours (1/4 per hour)

 

Equation:  1/6+1/x=1/4

 

Now, solve for x.

 

Let me know if you need any more help!

 

Norm.