Question 829882
R1 R1
R1 R2
R1 B1
R1 B2
R2 R1
R2 R2
R2 B1
R2 B2
B1 R1
B1 R2
B1 B1
B1 B2
B2 R1
B2 R2
B2 B1
B2 B2
Those are the sixteen possible choices for the four balls. 
4 of those choices show 2 red balls.
{{{P=4/16=1/4}}}