24, 42, 60
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6 significant digits.
By doing the math in your head on a piece of paper or with your fingers. You should know 3, 6, and 9 are multiples of three, but for 2-digit numbers or larger you can add the digits and if they equal 3, 6, or 9 then they are multiples of 3. For example, 15 would be 1 + 5, which is 6, so it's a multiple of 3.
1532354351
14 and 814-8=6 14+8=22
181