Check Boxes change value of a calculation

I'm trying to work on a fillable PDF, but I'm new to JavaScript and have only don the bare minimum, I have been asked to do this on the PDF and I can't figure it out:

Youth Registration Fees: equals number put in text box(# of paid youth) x ($5.50 x the number of months chosen in #18). UNLESS Box 15 or 17 are checked. If Box 15 is checked the formula changes to the number put in the text box(# of paid youth) x ($2.50 the number of months chosen in #18).  If Box 17 is chosen, the formula changes to number put in text box(# of paid youth) x ($3.50 x the number of months chosen in #18)