There are 400+ Excel functions. You don't want to learn all of them because most of the functions won't be useful for your line of work.

You need to go through the description of each function and shortlist the most relevant for you. Then you can take help of Google search to dig deeper into their usage.

Most commonly used functions can be:

1. Vlookup
2. Index & Match
3. Rows
4. IF and Nested IFs
5. Sumproduct
6. Sum
7. Choose
8. Date
9. Days
10. Trim
11. Right, Left, Mid, Upper, Lower
12. Concatenate
13. Substitute
14. Randbetween
15. ISERROR
16. ISNUMBER
17. ISTEXT
18. COUNT, COUNTA
19. COUNTIF, COUNTIFS
20. ISEVEN, ISODD

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