private changevalue(a as object, b as error) handles checkbox1.checked, checkbox2.checked, checkbox3.checked 'and on, ...
   const MyDiscount as float=0.9F
   const TshirtPrice as float=5.0F
   dim total as integer=0
  
   if checkbox0.checked=true then total+=1
   if checkbox1.checked=true then total+=1
   if checkbox2.checked=true then total+=1
   if checkbox3.checked=true then total+=1
   if checkbox4.checked=true then total+=1
   if checkbox5.checked=true then total+=1
   if checkbox6.checked=true then total+=1
   if checkbox7.checked=true then total+=1
   if checkbox8.checked=true then total+=1
   if checkbox8.checked=true then total+=1

   price.txt=FormatCurrency(total*TshirtPrice*iif(total>1,MyDiscount^(total-1),1))
end sub

dim firstshirt as boolean
dim 2ndshirt
if checkbox1.checked = true then
if firstshirt = true then
2ndshirt = true
end if
end if

if checkbox2.checked = true then
if firstshirt = true then
2ndshirt = true
end if
end if

if checkbox3.checked = true then
if firstshirt = true then
2ndshirt = true
end if
end if

if checkbox4.checked = true then
if firstshirt = true then
2ndshirt = true
end if
end if

if checkbox5.checked = true then
if firstshirt = true then
2ndshirt = true
end if
end if

if checkbox6.checked = true then
if firstshirt = true then
2ndshirt = true
end if
end if

if checkbox7.checked = true then
if firstshirt = true then
2ndshirt = true
end if
end if

if checkbox8.checked = true then
if firstshirt = true then
2ndshirt = true
end if
end if

if checkbox9.checked = true then
if firstshirt = true then
2ndshirt = true
end if
end if

if checkbox10.checked = true then
if firstshirt = true then
2ndshirt = true
end if
end if

if firstshirt = true then
msgbox("5 dollars")
end if
if 2ndshirt = true then
msgbox("9 dollars")
end if
end sub