What is an easy way to calculate (1.04)^54?
I disagree. They are likely testing if you know the rule of 72. At a 4% growth rate, an investment should double every 18 years. So over the span of 54 years, the investment should 8x (at a 4% growth rate).
It is a question from the case, Gravestone Inc.
You are not expected to know how to do this type of mental calculation in an interview, so don't worry about it. You not expected to know financial heuristics.