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
![\dfrac{300}{3600}=\dfrac{1}{12}](https://tex.z-dn.net/?f=%5Cdfrac%7B300%7D%7B3600%7D%3D%5Cdfrac%7B1%7D%7B12%7D)
trees per second.
On the other hand, Mia can plant 1 tree in 13 seconds, so she can plant
![\dfrac{1}{13}](https://tex.z-dn.net/?f=%5Cdfrac%7B1%7D%7B13%7D)
trees per second.
Working together they can plant
![\dfrac{1}{12}+\dfrac{1}{13}=\dfrac{25}{156}](https://tex.z-dn.net/?f=%5Cdfrac%7B1%7D%7B12%7D%2B%5Cdfrac%7B1%7D%7B13%7D%3D%5Cdfrac%7B25%7D%7B156%7D)
trees per second, i.e. it takes them 156/25 seconds to plant a tree.
We conclude that they plany 100 trees in
![\dfrac{156}{25}\cdot 100=156\cdot 4=624](https://tex.z-dn.net/?f=%5Cdfrac%7B156%7D%7B25%7D%5Ccdot%20100%3D156%5Ccdot%204%3D624)
seconds