1. The problem is to find the index of an element in a list or array. 2. The index of an element is its position in the list, usually starting from 0 in programming. 3. The formula or method to find the index depends on the language or context, but generally it is: \texttt{index = position of element in list}. 4. For example, if we have a list $L = [a, b, c, d]$ and want to find the index of $c$, the index is 2 because $L[2] = c$. 5. Important rules: - Indexing usually starts at 0. - If the element is not found, the index is often -1 or an error. 6. Intermediate work example: Given $L = [10, 20, 30, 40]$, find index of 30. Step: Check each element: $L[0] = 10 \neq 30$ $L[1] = 20 \neq 30$ $L[2] = 30 = 30$ so index is 2. 7. Therefore, the index of 30 in the list is $2$.