You use 4 boxes of rice to serve 13 people. if you want to serve 39 people, what should you do?

You would use 12 boxes of rice.  This answer can be found since 13 goes into 39 3 times, so one would have to triple the recipe in order to make enough for 39 people.  If they are tripling the recipe they are going to need triple the rice, so 4x3 is 12, 12 boxes of rice.