找规律填数:(2,5,7,35)(4,6,3,36)(5,3,8,60)(?,2,3,21)

题目

找规律填数:(2,5,7,35)(4,6,3,36)(5,3,8,60)(?,2,3,21)
请问问号应该是几啊

答案

7
2*5*7/2=35
4*6*3/2=36
5*3*8/2=60
7*2*3/2=21