Function in Combo Box…

Hi, I was making a Tour Program on Excel. I have created two Combo boxes…
One is Titled “No. of Adults” and the second one is titled “No. of Children”. And both of the combo boxes have same values in them…which are: 0,1,2,3,4
In the Tour program, you are not allowed to have more than 4 people. So what i’m trying to do is, i’m trying to put a function in my combo boxes so if i choose “1” in “No. of Adults”, the values in the second Combo box will change to “0,1,2 & 3”. Meaning because i chose “1” No. of Adult, i can have “0”,”1″,”2″, or “3” No. of Children for the tour.
But i can’t figure out how to do that, Please help. Thanks.