Answer:
25 in by 45 in.
Step-by-step explanation:
The length of the box will be x + 20 - 10 - 10 = x
The width of the box will be x - 10 - 10 = x - 20
The height of the box will be 10
So the volume = length x width x height
= x(x - 20)10 = 1250
= 

= 0
10(x + 5)(x - 25) =0
x = -5 x = 25
- 5 must be discarded since a length cannot be negative.
Therefore x = 25 in = width of piece of cardboard
x + 20 = 25 + 20 = 45 in = length of piece of cardboard