A carpenter takes 6 minutes to saw a log into 3 equal pieces. Working at the same speed, how long will he take to saw a similar log into 6 equal pieces?

Making 3 pieces needs only 2 cuts, so one cut takes 6 ÷ 2 = 3 minutes. Making 6 pieces needs 5 cuts, so the time is $5\times 3=15$ minutes. The trap is 12 minutes: it doubles the 6 minutes because the number of pieces has doubled, but the number of cuts is always one less than the number of pieces, so cuts do not double when pieces do.