1. 90 students form 9 groups of 10 students each. 2. Hence, 9 chaperons are needed. 3. You just have 3 teachers as 3 chaperons. 4. Hence, you need 6 additional chaperons, and it is your ANSWER. 5. Regarding a visual model, take a page of paper, a pen and/or a pencil and do it PLEASE on your own. 6. Since I helped you to solve so critically important and difficult problem, do not forget to post your "THANKS" to me.