The arithmetic mean of five observations is 16. If one more number is added to the collection, the mean of six numbers becomes 21, find the value of the sixth number.

Frank has a devised a formula for his catering business that calculates the number of meatballs he needs to prepare. the formula is m=4a+2c, where c=number of children; m=number of meatballs; a=number of adults. how many meatballs are required for a party of 10 adults and 5 children?