Asap! one construction company can do a certain job in ten days and a second construction company can do the same job in fifteen days. how many days will it take to complete the job if the two construction companies work together?

Add 10+15 which is 25. Then divided by 2 which is 12.5, so if I am right it should take 12 and a half days to complete.

1. find the coordinates of b, c, e and f
b(-2,4)
c(1,1)
e(4,4)
f(6,2)

2. use the formula:
m=y2-y1/x2-x1

3. slope of bc
m=1-4/)
m=-3/3
m=-1

4. slope of ef
m=2-4/6-4
m=-2/2
m=-1

5. so both of the line segments have the slope of -1, have a nice day : d