В коробке лежат красные и желтые шарики. Сколько красных шариков в коробке, если...

$N_{total}=N_{yellow}+N_{red}=20+N_{red}$

$Probability\ (to\ pick\ up\ the\ red\ one)= \frac{N_{red}}{N_{total}} = \frac{N_{total}-N_{yellow}}{N_{total}}=\\ =1- \frac{N_{yellow}}{N_{total}}=1-\frac{{20}}{N_{total}}= \frac{3}{8}$

$1-\frac{{20}}{N_{total}}= \frac{3}{8} \\ 1- \frac{3}{8} =\frac{{20}}{N_{total}}\\ \frac{5}{8} =\frac{{20}}{N_{total}}\\ \frac{1}{8} =\frac{{4}}{N_{total}}\\ N_{total}=8*4=32$

Answer: $32$