3. Three workers, A, B, and C, can complete a certain task in 10, 5 and x hours respectively. A starts working alone and 2 hours later B joins. After another 2 hours joins C. After that A, B, and C together complete the task in 15 minutes. What is the value of x?A. 1
B. 1.25
C. 2
D. 2.5
E. 4
A alone can complete the task in 10 hours, in 1 hour he can do 1/10 of the task
B alone can complete the task in 5 hours, in 1 hour he can do 1/5 of the task
C alone can complete the task in x hours, in 1 hour he can do 1/x of the task
A and B combined can do (1/10 + 1/5) = 3/10 of the task in 1 hour hour
A, B and C combined can do (1/10 + 1/5 + 1/x) = (3/10 + 1/x) of the task in one hour
As per question, A starts the work and works for 2 hours so he completes 1/10 * 2 of the task i.e. 1/5 of the task
Then A and B work for next two hours i.e they complete 3/10 *2 = 3/5 of the task
Remaining task = 1 - (1/5 + 3/5) = 1/5
This remaining task is done by A, B and C combined in 15 mins.
So 1/5 = (1/10 + 1/5 + 1/x)* 15/60, that gives x =2 (option c)