Use upto insted of ranges to make arrays
Use remainder and nonzero? methods
Use drop insted of slices to drop the first element
Pass block as an argument to make the code more concise
Rewrte prime factors, making it faster
Remove unused method prime_divisors
Use map instead of each_with_object
Add blank line after return statement to separateh it
Rememeber there is String#to_i
Use hash with default values
Fix average to work for empty arrays
Rewrite Array#drop_every using learned methods
Make Array#combine_with pass all tests