Answer:
8 units
Explanation:
It can help to consider the divisors of 40:
40 = 1×40 = 2×20 = 4×10 = 5×8
The pair of numbers in this list that differs by 3 is 5 and 8. The length is 3 units longer, so the length is 8 units.
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Alternate solution
Let L represent the length. Then the width is (L-3), and the area is the product of length and width:
A = L(L-3)
40 = L^2 -3L
We can "complete the square" by adding the square of half the L coefficient.
40 +1.5^2 = L^2 -3L +1.5^2
42.25 = (L -1.5)^2 . . . write the right side as a square
6.5 = L -1.5 . . . . . . . take the positive square root
8 = L . . . . . . . . . . . . add 1.5
The length of the rectangle is 8 units.