by 15 percent/ at 15 percent有什么区别

by 15 percent/ at 15 percent有什么区别
by 15 percent/ at 15 percent有什么区别

by 15 percent/ at 15 percent有什么区别
by 15 percent指的是 （上升或下降了）15%
at 15 percent指的是 （上升或下降到）15%
至于是上升还是下降,这要看上下文的语境了.