The initial population is 3900 , and it grows with a doubling time of 10 years . what will it be in 14 years?with the explanation of this question

Im going to try my hardest to explain this simply because its kinda hard but in 10 years the town would grow by 100% so take 3900 and figure out what 40% of 3900 would be (3900 x .40 = 1560) so add 1560 to the other 100% which is 5460 and then add that to the original population which is now 9360. So the population in 14 years would be 9360. The reason I went into depth is because most teachers want an explanation. and what grade is this???