Dhivya is building her new house. The family room of the house is \(20\) \(ft\) by \(28\) \(ft\). She wants to cover the entire floor with identical square tiles without cutting any tile. To use the fewest possible tiles, she wants the side length of each tile to be a whole number of \(\text{feet}\).
(a) Which size of square tile should Dhivya buy?
(b) How many tiles of this size are needed to cover the entire floor?