A rule can be used to quickly divide a board into equal parts. Lay the rule on the board with the start of the scale against one edge. Then angle the opposite end to a number that is easily divided by the number of parts you want. (If you want to divide a board into three equal parts you might use 9 and mark the board at 3 and 6.) Likewise, if you need four equal parts, angle the rule to numbers divisible by four, like 8, 12 or 16.
Check your ducts for air leaks. First look for sections that should be joined but have separated and then look for obvious holes. Consult with a professional about repairing duct leaks. Safe duct repairs require a licensed heating, ventilating, and air-conditioning contractor.