The area is found by multiplying the length by the width.
The original area = 60 x 40 = 2,400 square feet
He wants to make it 1/2 that size, so the new size would be 2,400 / 2 = 1,200 square feet.
To find the length divide the new area by the new width:
1,200 / 30 = 40
The new length should be 40 feet.