To do this, change into common denominators. The common denominator is 21 so change the problem from 1/3 + 1/7 to 7/21 + 3/21. This gives you 10/21. It is already in simplest form.
I believe the answer is 8
So I looked it up and it says something about 8 hours
If your question is how many positions are possible, then the answer is 24.
We find this by thinking of how many people can be in each spot. If there are 4 spots each person can stand (back left, back right, front left, front right), then there are 4 possible choice for the first spot (mom, dad, son, daughter), then 3 for the next spot (since 1 is already in a spot), then 2 options for the next spot, and then 1 option for the last spot. In other words, the answer is 4x3x2x1, or 4 factorial.
