A workshop's 12 tailors turn out 240 shirts in 5 days. For a fresh order of 480 shirts the strength is raised to 15 tailors, each stitching at the pace of before. How many days does the fresh order need?

12 tailors over 5 days is 60 tailor-days for 240 shirts, so one tailor stitches 4 shirts a day. 15 tailors therefore stitch 60 shirts a day, and 480 ÷ 60 = 8 days. 10 days doubles the 5 days for the doubled order but forgets the extra tailors. 4 days adjusts for the extra tailors $(5\times 12/15)$ but forgets the doubled order. 12.5 days doubles for the order and then multiplies by 15/12 instead of 12/15, putting the workforce ratio the wrong way up — more tailors must mean fewer days.