6 people can plant 12 trees in 3 hours is our assumption
this means 1 person can plant 2 trees in 3 hours (12/6=2)
this means 6 people can plant 4 trees in 1 hour (12/3=4)
this means 6 people can plant 8 trees in 2 hours, 12 trees in 3 hours, 16 trees in 4 hours, 20 trees in 5 hours and 1 more tree in 1/4 hour for a total of 21 trees in 5 1/4 hours
so...
6 people can plant 21 trees in 5 1/4 hours
line two says 1 person can plant 2 trees in 3 hours
how many trees can 1 person plant in 5 1/4 hours ?
(5 1/4)/3=(21/4)/3=(7/4) hours, (7/4)*2=7/2=3 1/2 trees
1 person can plant 3 1/2 trees in 5 1/4 hours
2 people can plant 2x(3 1/2)=7 trees in 5 1/4 hours
so far we have...
2 people plant 7 trees in 5 1/4 hours
6 people plant 21 trees in 5 1/4 hours
therefore...
8 people plant 28 trees in 5 1/4 hours
note that you could have gotten the answer from line two...
1 person, 2 trees, 3 hours
1 person, 3 1/2 trees, 5 1/4 hours
8 people plant 8x(3 1/2) trees in 5 1/4 hours
8 people plant 28 trees in 5 1/4 hours
