Answer: 8 batches
===================================================
Step-by-step explanation:
L = length = 10 ft
W = width = 5 ft
P = perimeter
P = 2*(L+W)
P = 2*(10+5)
P = 30
The perimeter of his bedroom is 30 feet. He already has 6 feet done so far, so he needs 30-6 = 24 more additional feet of border.
Let x = number of batches, where 1 batch is 3 feet of border material
1 batch = 3 feet
2 batches = 6 feet (multiply both sides by 2)
and so on
In general,
x batches = 3x feet
3x feet = 24 feet
3x = 24
x = 24/3
x = 8
He needs to buy 8 batches of border material.