Question 945318
There are 16 possible outcomes
RRRR
RRRW
RRWR
RRWW
RWRR
RWRW
RWWR
RWWW
WRRR
WRRW
WRWR
WRWW
WWRR
WWRW
WWWR
WWWW
Of those, 4 outcomes have exactly 3 red balls (RRRW,RRWR,RWRR,WRRR).
So,
{{{P=4/16=1/4}}}