F.4 Mathematics equations

Let Susan takes x days to finish the work alone.
So Mary takes (x + 5) days to finish the work alone.
In one day, Susan can finish 1/x part of the work.
In one day, Mary can finish 1/(x + 5) part of the work.
So in one day, if working together, they can finish 1/x + 1/(x + 5) part of the work.
Since they can finish the work in ( x - 4) days, that means they can finish 1/(x - 4) part of the work in one day if working together.
Therefore, 1/x + 1/(x + 5) = 1/(x - 4)
(x - 4)(x + 5) + x(x - 4) = x(x + 5)
x^2 + x - 20 + x^2 - 4x - x^2 - 5x = 0
x^2 - 8x - 20 = 0
(x + 2)(x - 10) = 0
x = -2 (rej) or 10.
So Susan can finish alone in 10 days.

Susn 獨做需x天, Mary獨做需x+5天
1/x+1/(x+5)=1/(x-4)
(x-4)(x+5)+x(x-4)=x(x+5)
x^2+x-20+x^2-4x=x^2+5x
x^2-8x-20=0
(x-10)(x+2)=0
x=10,-2(rejected)
answer: 10 days