I got it.
I multiplied 210,000 by 4 and got 840,000.
Then subtracted 155,000 (685,000)
Then subtracted 180,000 (505,000)
Then subtracted 365,000 (140,000)
Back of book says:
Berne (the Bear) is one of the four largest cities in Switzerland and is the capital. The average population of the four largest cities is about 210,000. If the population of Geneva is about 155,000 and if the population of Basel and Zurich are approximately 180,000 and 365,000, respectively, what is the population Berne?