Generally speaking, if a barber cuts his hair in chronological order: from short to long.

That is, the sequential processing time of customers 1, 2, 3, 4 and 5 is the shortest.

But the hairstyles of the two hairdressers in this question are 1, 3, 5; Rule 2.4. Jump from short to long according to time.

The shortest time is:

10 * 3+15 * 2+24 *12 * 2+20 *1=128 (dot)