In a rectangle, there are two lengths and two widths with the same measure.
If one length is 25cm, it has to have another length of 25cm. Thus, the rectangle must have 2 widths with the same value.
The formula to find the perimeter is the sum of all sides, the rectangle has 4 sides (2 lengths, 2 Widths).
Perimeter (Rectangle) = Length + Length + Width + Width = 90cm
It's the same as writing P = 2 x Length + 2 x Width = 90cm.
Now to find the width, just substitute the length value in the formula:
-> P = 2 x 25 + 2 x Width = 90cm
-> 50 + 2 x Width = 90cm
-> 2 x Width = 90 - 50
-> 2 x Width = 40
-> Width = 40/2
-> Width = 20cm