What is the next number of the following sequence: 15, 60, 135, 240, ?

Answer 1

15 60 135 240 375
60 - 15 = 45
135 - 60 =75 (45+30=75)
240-135 = 105 (75+30=105)
Now, 105+30=135
Then, 240 + 135 = 375

Answer 2

15x(1X1)=15
15x(2x2)=60
15x(3x3)=135
15x(4x4)=240
15x(5x5)=375
Ans : 375