Iterating is usually better practice than indexing if possible, as it has fewer failure points (though in this case it's trivially fine):
for (i, n) in arr.iter().enumerate() {

till 1 will serve 2 then 10

till 2 - 3 then 2

total time - 12

There is no need for complicated concepts. Try to solve simple case on paper first. E.g ([1, 1, 2, 3], 2) == 4