Question 190
At a congress all attendees are either a mathematician or a biologist and there is no one that is both. The mathematicians all know each other and each of them knows four of the biologists. The biologists also all know each other and each of them knows nine of the mathematicians. It turns out that every mathematician knows twice as many people as every biologist. (If person $A$ knows person $B$, then person $B$ also knows person $A$.)
How many mathematicians are at the congress?
How many mathematicians are at the congress?