Query & Calculation

[Sereen]

Greetings,

i build a query, called "Down Payments", has down payment, First
installment, sencond installment

so First installment: [courses.cost]-[student/courses.down payment]

second installment: [courses.cost]-[student/courses.down
payment]-[first installment]

and for now every thing is working, the problem is:

example:

cost:2000
downpayment:1000
in the First installment column the this number will appear
automatically : 1000

but the user may not pay 1000 so i want to edit it but i can, what i
want is

when the number is 1000, i may change it to 500 and in the second
installment column the number 500 shows up automatically.

if someone can help

Regards