Analyzing a query plan for a query with TWO predicates, but index only has ONE
The optimal index for that query is INDEX(faculty, wam), in that order.

MySQL does not (in most situations) use two different indexes.  However, it will use a 'composite' index to good use in some situations, such as this.

Without knowing how many rows have Arts and home many have &gt; 75, you cannot accurately conclude which of INDEX(wam) or INDEX(faculty) would be better.  Nor can the Optimizer, since it's statistics gathering is not very good.  SHOW INDEXES and EXPLAIN SELECT provide numbers that come from the same statistics, so you won't necessarily come to the optimal conclusion.

A "Hash" index (which InnoDB does not have) is essentially no better than a BTree index.  A Hash index is useless for a range (eg, &gt; 75).  (Hence, MySQL uses BTrees, and does not provide Hash indexes.)

Another point:  If more than a small percentage of the table has wam &gt; 75, INDEX(wam) will be shunned; scanning the entire table is likely to be faster than bouncing between the index's BTree and the data BTree.  Again &gt; 75 and 60 pages and 10 and 100 give no good clues of which path the Optimizer should take.  The answer to the quoted question is "it depends" or "not enough info".

You could tackle the question by presenting two answers:  One where very few rows have wam &gt; 75, and one where most meet that predicate.

Has your instructor had any experience using databases, or only textbooks?

More discussion on devising optimal indexes:  http://mysql.rjweb.org/doc.php/index_cookbook_mysql