Make multiplication & then addtion by selecting an item from dropdownlists

hi to all,
i need a help to make  calculations
i want to make multiplication & then addtion for it by selecting the quantity value from the dropdownlist.
There are 3 dropdownlists   ADULT,CHILD,INFANT.
Each dropdownlists has 5 items or quantity values i.e 0,1,2,3,4.
Now i need to multiply the cost of the ticket by retriving from the sql database table reservation from fare column with the selection of item in each dropdownlists.Then later this 3 dropdownlists values should added together to a single label.
Example:
Each ADULT,CHILD,INFANT tickets cost 2000,1000,1500
ADULT[2000]x3[quantity selected from dropdownlist]= 6000[value obtained by multiplication]
CHILD[1000]x2[quantity selected from dropdownlist]= 2000[value obtained by multiplication]
INFANT[1500]x2[quantity selected from dropdownlist]=3000[value obtained by multiplication]
---TOTAL COST VALUE OBTAINED AFTER SELECTION---= 9000 [value obtai ...