$20 \div 5 = 4$ intervals on each side  $4+1=5$ trees on each side  $4\times5-4=16$ 
Harry planted some trees around a $20$m by $20$m square garden. If there are trees planted at each of the $4$ corners, and the distance between every $2$ trees is $5$m, at least how many trees did Harry plant? 
1. $$16$$ 
2. $$18$$ 
3. $$20$$ 
4. $$22$$ 