quadratic equation的文字題

1.Let width of walk = x. Then width of garden with the walk = (2x + 3) and length of garden with the walk = (2x + 7). Since area of garden itself is 3 x 7 = 21. Then the difference will be the area of the walk which is 11. That is
(2x+3)(2x+7) - 21 = 11
4x^2 + 6x + 14x + 21 - 21 - 11 = 0
4x^2 + 20x -11 = 0
(2x +11)(2x -1) = 0
x= -11/2 (rej because x cannot be negative) and x=1/2 = width of walk.
2.
Let John takes x days to finish the job alone.
So Mary takes (x+5) days to finish the job alone.
In one day, John can finish 1/x of the job.
In one day, Mary can finish 1/(x+5) of the job. So if working together in one day, they can finish 1/x + 1/(x+5) of the job.
Since the job can finish in 6 days, then in one day they finish 1/6 of the job.
Therefore,
1/x + 1/(x+5) = 1/6
6(x+5) + 6x = x(x+5)
6x + 30 + 6x = x^2 + 5x
x^2 - 7x -30 = 0
(x + 3)(x-10) = 0
x= -3(rej. because days cannot be negative) and x = 10.
Therefore, John can finish in 10 days alone, Mary can finish in 15 days alone.

good job