Assuming that it takes X days to complete the task, C helps A to repair Y days. Then there is the equation:

(x/ 10)+(2/30)+(y/ 15)= 1①

(x/ 12)+(x-y-2)/ 15 = 1②

Simplified by (1):

3x+2y=28 ③

Simplify from ②:

9x-4y=68 ④

Get ③*3-④.

10y= 16

y= 1.6

Substitute y= 1.6 into ③ to get x= 124/ 15=8 and 65438+4/05.

So it took 8 65438+4/05 days to complete the task, which is 8 days, 6 hours and 24 minutes.

The answer is over, I don't know if it's right. It seems that the problem-solving process is no problem!