We have to find out the number of trees planted per unit rate of time.
Kyla can plant 300 trees in 1 hour, so she can plant 300 trees in 3600 seconds. This implies that she can plant
trees per second.
On the other hand, Mia can plant 1 tree in 13 seconds, so she can plant
trees per second.
Working together they can plant
trees per second, i.e. it takes them 156/25 seconds to plant a tree.
We conclude that they plany 100 trees in
seconds