I’m struggling to get to grips with recursions. I can get some grip on the ide but on the actually exercise I really don’t get how (arr, n) == (arr, n --1)*arr[n-1].

The idea is to multiply all the elements in the array given by n together.
If n was say 3, then this would be (3-1)*(3-1) = 4. Or 9 if n = 4, or 16 if n = 5.

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you are missing the function
the point is that in one case the function multiply the first n elements of the array together, in the second one it multiply the first n-1 elements, and then the nth is multiplied to that result, both gives the same result

recursion is calling the function inside itself but with a smaller argument
you can calculate the product of the first n elements in an array if you first get the product of the first n-1 element and then multiply to it the nth