Question 404887
The first ball hit is yellow is {{{4/20=0.2}}} (because in box 4 yellow balls, (16+4)=20 balls)
both balls are yellow is the first ball is yellow and the second ball is yellow. Probability, that the first ball is yellow {{{4/20=0.2}}}, the second ball is yellow (after the first ball was yellow)is {{{3/19}}}
Both balls are yellow {{{0.2*(3/19)=0.0316}}}
Answer: a) 0.2, b)0.0316