Answer:
840
Explanation:
The 3 by 3 grid below shows nine 1 cm x 1 cm squares and uses 24cm of wire
24 cm of wire
is 3 (3 + 1) + 3(3 + 1)
= 3 x 4 + 3 x 4
= 12 + 12
= 24 cm
length of wire is required for a similar 20 by 20 grid
= 20(20 + 1) + 20(20 + 1)
= 20 x 21 + 20 x 21
= 420 + 420
= 840 cm