The answer is (a) 8m
This is because, if the length is 3 times as long as the width, you would multiply the numbers by 3 to give you a length.
8×3=24 16×3=48
We can stop at this point as we know the perimeter is 64 and to work out the perimeter you add up all the sides. 48+48+16+16 is larger than 64 so the only smaller number is 8. We can check this is correct by adding up the numbers.
8+8+24+24=64
We now know that the width of the field is 8m.
