luke has 78 books on his to read list. he has already read 10. he plans to read 4 a month. how long will it take luke to read all of his books on his to read list?
Luke has 78 - 10 = <<78-10=68>>68 books left to read.
At a rate of 4 books per month, it will take Luke 68 / 4 = <<68/4=17>>17 months to read all of his books on his to read list. Answer: \boxed{17}.