A bag contains 7 blue balls and 5 yellow balls. If two balls are selected at random, what is the probability that none is yellow?

Blue balls= 7 

None-ball out of two are yellow Yellow balls = 5 

Both balls are blue

$\therefore$ Total balls= 12

$\therefore$ P (both blue balls) $=\frac{7}{12}\times \frac{6}{11}=\frac{7}{22}$