# How to design a control when you select any 3 of 4 parameters to get the remaining one

I am thinking from the UX perspective about implementing a control where there are four parameters (see example below). The user can select the values for any three of them to get the value of the fourth (parameters are related).

### Example

The 4 parameters are E (energy), V (voltage), I (current), t(time). When I select any 3 out of the 4 - the last one is obtained from the equation - E = V x I x t (energy = voltage x current x time).

• The simplest solutions is 4 inputs with good written logic? – Piotr Żak Jan 14 at 12:14
• The problem is that 4th input becomes an output when I have the rest 3 inputs. – vkrouglov Jan 14 at 12:42
• If you have 1,2,3 (4 automatic), when you have 2,3,4 (1 automatic), when you have 3,4,1(2 automatic) - You have 24 combinations - so i think the inputs should be identical - if there is one unknown in the equation - it doesn't matter which input. – Piotr Żak Jan 14 at 12:43
• I have found a very similar question discussed here: ux.stackexchange.com/questions/50511/… - I do not like the solutions there. Has to be a more intuitive way? – vkrouglov Jan 14 at 12:47
• A similar question: ux.stackexchange.com/q/60022/36883. It's about a calculator and seems almost the same. – jazZRo Jan 14 at 14:19

By offloading the work upfront, ask for the missing details (i.e output) 