What is the sum?

StartFraction x Over x + 3 EndFraction + StartFraction 3 Over x + 3 EndFraction + 2 Over x + 3 EndFraction
Five-thirds
StartFraction x + 5 Over x + 3 EndFraction
StartFraction x + 5 Over 3 x + 27 EndFraction
StartFraction 6 x Over x + 3 EndFraction

For the rule of 78, for a 12-month period, the last term in the sequence is 12 and the series sums to 78. for an 10 month period, the last term is and the series sum is . for a 15 month period, the last term is and the series sum is . for a 20 month period, the last term is and the series sum is

How to do this , and can some one explain

You are designing a rectangular pet pen for your new baby puppy. you have 30 feet of fencing you would like the fencing to be 6 1/3 feet longer than the width