It took Morty two weeks to build seven model airplanes. At the same rate, how many days will it take to build nine model airplanes?

It would take 18 days. If you build 7 airplanes in 14 days, that would mean that you build 1 airplane every 2 days. So you would just need to do 9•2, which equals 18, to find out the answer
Hope this helps :)