Capture data from number field

I need help. I made 2 fields , 1 is for quantity field and 2nd for check box…
I made this formula quantity*1.25$ + checkbox .

my quantity is for checkbox and i want to capture the value for this checkbox field from number field.. So when anyone select the quantity field as 4 number then the value for check box should be automatically 4 , if anyone select the quantity field 5 then value of check box should be automatically change to 5 .

Please help me ASAP