The width would be 1 inch.
Given: L = length = 5W; P = perimeter = 2(L+W) = 12 inches
Required: W = width;
Formula: P=2(L+W)
Solution:
12 in = 2 (L + W)
6 in = L + W
6 in = 5W + W
6 in = 6W
1 in = W
Cross check:
P = 2(L + W)
12 in = 2(5 in + 1 in)
12 in = 2(6 in)
12 in = 12 in