$4.$ Assume we use an index$-$join with R$2$ as the outer relation, and we have an index on R$1.$a$.$ Assume that the index fits in memory. Moreover, on average we get $5$ R$1$ tuples matching every R$2$ tuple.

a$.$ What is the cost of the join algorithm?

Here you might need to make assumptions i$.$e$.$ one or both tables are clustered $($or not$),$ or index is clustered or not, index in memory or not, etc.