Please help me answer this

Empty box + box + empty box fill in the box is equal to 30 how

Adeposit of 5500 at 6% for 3 years equals

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?