Answer:
It will take the first person 12 days and the second person x+12 or 20 days
Step-by-step explanation:
The equation for time to do a job is
1/A + 1/B = 1/C
where A is the time for person A to do the job alone
B is the time for person B to do the job alone
and C is the time for A and B to do the job together
Person A does it in x days and Person B does it in x+8 days and together they do it in 7.5 days
Substituting in
1/ (x) + 1/ (x+8) = 1/ 7.5
Multiply by (x)(x+8)(7.5) to clear the fractions
(x)(x+8)(7.5)(1/ (x) + 1/ (x+8)) = (1/ 7.5)(x)(x+8)(7.5)
Distribute
(x)(x+8)(7.5)(1/ (x)) +(x)(x+8)(7.5)( 1/ (x+8)) = (1/ 7.5)(x)(x+8)(7.5)
Cancel
(x+8)(7.5) + 7.5x = (x)(x+8)
Distribute
7.5x +60 +7.5x = x^2 +8x
Combine like terms
15x +60 = x^2 +8x
Subtract 15x from each side
15x-15x +60 = x^2 +8x-15x
60 = x^2 -7x
Subtract 60 from each side
60-60 = x^2 -7x - 60
0 = x^2 -7x - 60
Factor
0 = (x-12) *(x+5)
Using the zero product property
x-12=0 x+5=0
x=12 x=-5
Since the number of days cannot be zero
It will take the first person 12 days and the second person x+12 or 20 days
