I recently interviewed for Google. My first round of interview had this question. Which should I solve within 40 mins

You work in Human Resources of a large company, and during a recent review, you come to realize that some managers might seem underpaid.
To ensure fairness, you come up with an impartial rule:

A manager is underpaid if and only if their salary is less than the average
of the salaries of all their direct and indirect reports.
For example, consider the following:

      A($100)
        |

+-B($100) +- C($200)
     |          |
E(200)        D($60)

Here A is underpaid, since the average salary of their reports is ($100+$200+$60+$200) / 4 = $117.5whichismorethan$100.

Question: Given an organization tree, return the count of underpaid managers.

I solved it using DFS , Dry run failed some testcases. Interview result is not yet announced.