As tahir suggested, Excel on a smartphone seems like the simplest solution to me.

Cell A1: [where you enter your number]
Cell A2: Locked, not changeable without unlocking, contains this formula: =A1*A3
Cell A3: Locked, hidden, contains your Mystery Formula™.

Call up Excel on the phone, the cursor is already on A1, enter your number and the result appears below it. Or use A1, B1, and C1 if you prefer a horizontal layout.

tanstaafl.