51. I checked the comments but I still didn't understand it, sorry :/
54.
(43+45+47+49)=184
They are all odd and they are in consecutive order (one after another)
It would take only one full bus to carry only girls.
Check the picture below.
let's recall that the point of tangency with the radius chord is always a right-angle.
What do you think the answer to the question is?