A (alone) completes the task in 10 hours, so in 1 hour he does 1/10 of the task

B (alone) completes the task in 5 hours, so in 1 hour he does 1/5 of the task

C (alone) completes the task in x hours, so in 1 hour he does 1/x of the task

A and B combined do : 1/10 + 1/5 = 3/10 of the task in 1 hour hour

A, B and C combined do: 1/10 + 1/5 + 1/x = 3/10 + 1/x of the task in one hour

A starts work and works for 2 hours, so he completes 1/10* 2 = 1/5 of the task

Then A & B work for next 2 hours and complete 3/10*2 = 3/5 of the task

Remaining uncompleted part = 1 - (1/5 + 3/5) = 1/5

This remaining task is done by A, B and C combined in 15 mins.

1/5 = (1/10 + 1/5 + 1/x)*15/60

so x =2

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...Thanks for KUDOS