How? I'm thinking this is a statistics class assignment. You could use Excel GoalSeek in a more general case, but I think that probably the tables solution is the one wanted here because the answer comes out exactly to a nice round number.

Yes, it looks like assignment. But they should stop using old tables in assignments! it doesn't make sense.
like they don't use slide rules anymore ...

I don't hate tables. Part of the course typically is learning how to read charts and graphs. The tables really aren't that complex and a student should be able to understand them.

I really don't hate tables, and I also liked to play with my grand father's slide ruler when I was a kid.
But I think it is important to learn to use the tables exactly as it is important to learn to use the slide rule.
Where is the next slide rule class?