A corridor was divided into equal 4 m parts, with 2 potted plants in each part. The SAME corridor was divided into equal 6 m parts, with 5 lanterns in each part. There were 24 more lanterns than plants. How long was the corridor?
You cannot compare "2 plants" against "5 lanterns" directly — they are counted over different lengths (4 m vs 6 m). Compare them over a length BOTH fit evenly: 12 m.
12 ÷ 4 = 3 parts × 2 plants = 6 plants.
12 ÷ 6 = 2 parts × 5 lanterns = 10 lanterns.
10 − 6 = 4 extra lanterns per 12 m. We need 24 extra altogether:
24 ÷ 4 = 6 lots of 12 m.
(Check: 72 m gives 18 × 2 = 36 plants and 12 × 5 = 60 lanterns, and 60 − 36 = 24 ✓)
The trap: comparing 2 plants against 5 lanterns directly and using a difference of 3. The parts are different LENGTHS, so the counts can only be compared over the same distance.