Hi,
I really don't know what to do i have been trying for weeks i need to do the following

If i enter say the number 1 it =$150 if i enter 2 it also = $150 but i need it to multiply so it add up to $300...but if i enter say 3 that = $200 plus multiply so that = $600
i really am very green i have been trying if and array but have no idea can some one help in small steps please?