Pass tests
Use pred and succ
Use Rational(1, i) instead of 1 / Rational(i)
Use String#chars insted of String#split('')
Use reduce instead of inject
Use reference to the to_i method in map
Handle the empty array in Array#average
Use reject instead of select in Array#drop_every(n)
Simplify combine_with
Use recursion and find in Integer#prime_factors
Use each_with_object in Array#frequencies