answers.txt

 1 x 12 = _ 12_   |    1 x  9 = _  9_
 2 x  4 = _  8_   |    2 x  5 = _ 10_
 3 x 11 = _ 33_   |    3 x  7 = _ 21_
 4 x  8 = _ 32_   |    4 x  5 = _ 20_
 5 x  7 = _ 35_   |    5 x 12 = _ 60_
 6 x  3 = _ 18_   |    6 x 11 = _ 66_
 7 x 10 = _ 70_   |    7 x  3 = _ 21_
 8 x  8 = _ 64_   |    8 x 11 = _ 88_
 9 x 10 = _ 90_   |    9 x  9 = _ 81_
10 x 11 = _110_   |   10 x 11 = _110_
11 x 12 = _132_   |   11 x  4 = _ 44_
12 x  4 = _ 48_   |   12 x  7 = _ 84_
 1 x 11 = _ 11_   |    1 x  3 = _  3_
 2 x  8 = _ 16_   |    2 x  6 = _ 12_
 3 x  5 = _ 15_   |    3 x 10 = _ 30_
 4 x  3 = _ 12_   |    4 x  9 = _ 36_
 5 x  4 = _ 20_   |    5 x 10 = _ 50_
 6 x  5 = _ 30_   |    6 x 12 = _ 72_
 7 x 11 = _ 77_   |    7 x  8 = _ 56_
 8 x  4 = _ 32_   |    8 x  5 = _ 40_
 9 x 11 = _ 99_   |    9 x  6 = _ 54_
10 x  4 = _ 40_   |   10 x  7 = _ 70_
11 x  9 = _ 99_   |   11 x 10 = _110_
12 x  3 = _ 36_   |   12 x  6 = _ 72_

math_answers.rb

#!/usr/bin/ruby

open('answers.txt', 'w') do |f|
  File.new('problems.txt', chomp: true).readlines.each do |line|
    m = line.match(/(?<a>\d+)(\D+)(?<b>\d+)(\D+)(?<c>\d+)(\D+)(?<d>\d+)/x)
    f.puts sprintf("%2d x %2d = _%3d_   |   %2d x %2d = _%3d_", m[:a], m[:b], (m[:a].to_i * m[:b].to_i), m[:c], m[:d], (m[:c].to_i * m[:d].to_i)) if m
  end
end