We can solve this question by using simultaneous equations (2 equations for 2 unknowns).
Let the width be w and the length by l. Write 2 equations from the given info.
l = 2w (length of the rectangle is twice that of width) (1)
(l + w) x2 = 24 (perimeter is 24) (2)
Substitute (1) into (2),
(2w + w) x 2 = 24
2w + w = 24/2
3w = 12
w = 4
Therefore, the width of the rectangle is 4cm.
