```ruby
require 'pp'
hash = Hash.new {|h,k| h[k] = [] }
(0..99).step(5).each do |num|
# XXX: 10 - 10%10 => 10
range = num - num%10
hash[range] << num
end
pp hash
{0=>[0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
10=>[10, 11, 12, 13, 14, 15, 16, 17, 18, 19],
20=>[20, 21, 22, 23, 24, 25, 26, 27, 28, 29],
30=>[30, 31, 32, 33, 34, 35, 36, 37, 38, 39],
40=>[40, 41, 42, 43, 44, 45, 46, 47, 48, 49],
50=>[50, 51, 52, 53, 54, 55, 56, 57, 58, 59],
60=>[60, 61, 62, 63, 64, 65, 66, 67, 68, 69],
70=>[70, 71, 72, 73, 74, 75, 76, 77, 78, 79],
80=>[80, 81, 82, 83, 84, 85, 86, 87, 88, 89],
90=>[90, 91, 92, 93, 94, 95, 96, 97, 98, 99]}
```