question: 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=<<10-2=8>>8 new trees a year
  After 10 years he has 8*10=<<8*10=80>>80 new trees
  So he had 80+50=<<80+50=130>>130 trees
  He lost 130*.3=<<130*.3=39>>39 trees
  That means he had 130-39=<<130-39=91>>91 trees
  #### 91