Emily,
First we need to calculate the painting "rate." A rate is something like miles per hour, that is to say an amount of something (i.e. miles) done in a unit of time (i.e. minutes). In this case our generic rate is painted-walls per minute per person. So, our base rate is 10 walls per 56 minutes per 5 people. Now treat every "per" as a division, so we can transform our rate into 10/56 of a wall, per minute per 5 people and then divide by 5 persons which means our rate is 2/56 (which reduces to 1/28) of a wall per minute per person.
Now we need to create an equation using this rate to calculate the number of minutes for 7 people to paint 8 walls. I find the best way is to keep track of the units. Generally, a rate x # of people x time = work, so our generic equation is
2 walls 7 persons # minutes 8 walls
-------------------- x -------------- x ---------------- = -----------
56 person - minute 1 1 1
2 walls x 7 persons # minutes 8 walls
-------------------- x -------------- = -----------
56 person - minute 1 1
14 walls # minutes [ 56 minute ] 8 walls [ 56 minute ]
------------ x -------------- x [ --------------- ] = ----------- x [ -------------- ]
56 minute 1 [ 14 walls ] 1 [ 14 walls ]
= 8 x 56 / 14 = 32 minutes
Finally, a reality check. It took 5 people 56 minutes to paint 10 walls, now we have more people (7) and fewer walls (8) to paint, so they should be able to do it faster and 32 minutes is less than 56 minutes, so our answer makes sense.
I hope this helps.