Question: Solve the following math problem: Tom plants 10 trees a year.  Every year he also chops down 2 trees a year.  He starts with 50 trees.  After 10 years 30% of the trees die.  How many trees does he have left?
Answer: 
He gets 10-2=8 new trees a year
After 10 years he has 8*10=80 new trees
So he had 80+50=130 trees
He lost 130*.3=39 trees
That means he had 130-39=91 trees