He started with 2*4=<<2*4=8>>8 trees
He plants trees for 15-10=<<15-10=5>>5 years
So he planted 5*4=<<5*4=20>>20 trees
So he had 20+8=<<20+8=28>>28 trees
After doubling them he had 28*2=<<28*2=56>>56 trees
#### 56