[In the case of the same quantity, the work efficiency is inversely proportional to the time spent. ]

If the time to play 240 strings is 1, then the time for Wang to play 240 strings is 5/6.

[The same efficiency, the time used is directly proportional to the quantity]

The time taken by Wang to compose 3 12 strings: (312/240) * (5/6) =13/12.

Yu Chen: (1312)-1=112.

It is known to be 2 hours long, so it is 2/(112) = 24 (hours).

Hour: 240/24 = 10 (string)