Merge hashes while adding values of identical keys

gistfile1.rb

a = [{'a' => 30, 'b' => 14}, {'a' => 4, 'b' => 23, 'c' => 7}, {'a' => 6, 'b' => 20, 'c' => 5}]
 
puts a.inject{|tot, new| tot.merge(new){|k,val_from_a,val_from_b|val_from_a+val_from_b}}

=> {"a"=>40, "b"=>57, "c"=>12}