The sequence 15,30,45,60 look like 1,2,3, each term is 15 times as large. so, the formula is 15n, where n represents the position of a term in the sequence. check the first four terms

fist on the left goes with the third on the right. second on the left goes with the fourth on the right. third on the left goes with the second on the right. the fourth on the left goes with first on the right