Button to do addition

I hope someone can help. I am making a weakest link screen to use with pupils when doing a quiz. So far i have an 11 frame movie clip which shows the amount of prize money. I have a correct button which moves the movie clip to show £100, £200 etc. I have a wrong button which when pressed takes the moviclip back to frame 1,which shows £0.

What i want to do is work on the ‘bank’ clip. When i press this i want it to add whatever sum they have reached and display it in a dynamic text box. e.g. if they have reached the frame in the money movieclip which has £400 highlighted, then 400 will appear in the text box.
I also want it to do addition, so that when the button is pressed the money movie clip might be in the frame which has £200 highlighted and then the total score box will show £600.

I think i want something along the lines

when the button is pressed,

if movie clip is in frame 1 then show 0 in text ( total score)
if movie lcip is in frame 3 then add 300 to total score
if moviclip is in frame 5 then add 500 to total score

But i have no idea about adding - i am rubbish at maths and avoid this section of flash.
Could anyone point me in the right direction. stockdill