Before giving this out, several identical questions would have been worked out in front of the class. So it's not a "figure this out from the worksheet" matter; it's a "recognize it's like the problems we did earlier".

It that context, it's a plausibly useful exercise; it's analogous to how I (still) total and multiply numbers in my head: determine the similar problem that gets to either a nice round number, or a memorized result -- then adjust for the remainder.