Trunks ($10$ m long) and cars ($5$ m long) go on a single lane bridge. There must be a gap of at least $20$ m after each trunk and a gap of at least $15$ m after each car. Trunks and cars travel at a speed of $36$ km/h. If cars and trunks go alternately, what is the maximum number of vehicles that can use the bridge on one hour?
- $1440$
- $1200$
- $720$
- $600$