Calculations in the cost/amount field

Calculations in the cost/amount field: There is a pretty cool feature in QuickBooks that on the invoice, in the Amount field it will allow you to do a calculation, just type in the expression 35*1.3 and it will put the value in the field. This is nice because you don't need to go get a calculator on the screen or device on your desk to do the math. Not a coder/programmer in anyway, don't know how to make it work, just know it is very nice for the user.