What are the two anchor percentages you build everything else from?

10% (move the decimal one place left) and 1% (move it two places) — every other percentage is a combination of these.

• 10% of 340 = 34.0 (decimal shifts one place).
• 1% of 340 = 3.40 (two places).

From those, 5% is half of 10% (17), 20% is double (68), 15% is 10% + 5% (51), 2% is double 1% (6.8). Almost any percentage is a quick sum of 10%s, 5%s, and 1%s.