A - 10 hours to complete the work

B- 5 hours to complete the work

C- X hours to complete the work

Take the LCM of 10,5,x = 10x (total work)

Hence A rate of work = 10x/10 = X units/hour

B rate of work = 10x/5 = 2x units/hour

C rate of work = 10x/x = 10 units/hour

therefore

X*2 +(2x+x)*2+(2x+x+10)*1/4 =10x

this gives X =2 (quick and easy)

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