ANSWER: $45!
Explanation: Okay so first, there are 12 friends and $15 per 4 of each friend! First we will make it easier by finding the original cost of 1 friend or a term called the mean! So to find the mean we will divide 15 by 4 which is ($3.75) so that means for one person it’s $3.75 so now that we have the mean we can check if it will give us a total of 15! So now 3.75 x 4= 15 so our mean is correct! Now it is easier! We can multiply 3.75 by 12 and that is 45! So your answer would be the group of friends will pay ($45) this is the more sufficient way but another more elementary style way is to do 12 divided by 4 which is 3 then next you will do, 15 x 3= 45 because, if there are 12 friends and $15 per four friends we will have to divided the 12 by 4 to find how much more groups of 4 friends needed! So after we got our total 3 we needed to multiply it by 15 because $15 is per 4 friends so we only needed three groups of 4 friends to get our answer!!
Hopefully this was not confusing in any way! Enjoy your day :)