Is there a way to lock a number box until radio button value activated

Thank you for all your information so far, very very helpful indeed. I'm completely new to Java Script so it is starting to sink in very slowly but this is a bit much, simple calculations is about as far as I have reached.

I'm using a exclusionary group (you can only check one, I beleive this means!) of radio buttons 1,2 ,3 for each participants 1 to 17 in a list.
After the third radio button value of £11.50 (which now works thanks to you guys as below) there is another box in which the participant may add an additional order of 1 or more @ a value of £3.00 each but only if they have checked radio button 3 @ £11.50. Is there a way of setting this box so no number may be added untill check box 3 has been checked. Then this total would be added to the total from the radio button total.

Also is there a way to add up each list of radio buttons for a total at the bottom from the 17 participants eg total of value 1 @ 5.25, total of value 2 @ 9.00, total of value 3 @ 11.50

var a = this.getField("partcipant_1_checkbox").value;
if(a == 1)
  event.value = 5.25; // 
else if (a == 2)
    event.value = 9.00; // 
else if (a == 3)
    event.value = 11.50; //

Rusty