Can anyone please provide clarification as to when we should use the permutation formula and when we should use the combination formula? I'm so confused and I only have 3 days until my test!
I was trying to solve this problem:
The telephone company wants to add an area code composed of 2 letters to every phone number. In order to do so, the company chose a special sign language containing 124 different signs. If the company used 122 of the signs fully and two remained unused, how many additional area codes can be created if the company uses all 124 signs?
I used the combination solution for this because we are trying to find the number of combos for 2 slots out of 122 and 124 different options. However, the solution just 124^2-122^2. I don't understand. Please help!
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