60% is really 60/100. You know that 126/x equals 60/100. this is a simple algebra problem. (see your textbook)
You then take 60 times x(the unknown value) and make it equal to 126 times 100 or 12,600. So 60x equals 12,600. To solve for x you must isolate it on one side of the equation. To do that you can divide both sides by 60. (60 divided by 60 equals 1) and 1 times anything equals itself.
60 x divided by 60 equals 1x or just x. 12,600 divided by 60 is equal to 210. So now you know that 126/210 equals 60/100. 210 is your total. You can now solve for 40% or just intuitively say that if 126 is 60% the other 40% (60% and 40% equal 100%) equals the remainder or 210 minus 126. This gives you an answer of 84.
Hope this helps. Remember math can be fun, it's just like solving a puzzle.