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.
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?