Last week, Ellen bought 4 apples for $2.40. %C2%A0This week she bought 9 apples for $5.40. %C2%A0If the cost per apple remains the same, how many apples could she buy next week?

The cost per apple is constant at $2.40/4 = $<<2.40/4=0.60>>0.60 per apple.

Next week, Ellen could buy $5.40/$0.60 = <<5.40/0.60=9>>9 apples. Answer: \boxed{9}.