\[a = 50 - красных;\]
\[b = 100 - белых;\]
\[c = 150 - желтых.\]
\[1)\ N = C_{300}^{3}.\]
\[2)\ N = 50 \bullet 100 \bullet 150.\]
\[3)\ N = C_{50}^{2} \bullet C_{100}^{2} \bullet 150.\]
\[4)\ N = C_{50}^{3} \bullet 100 \bullet C_{150}^{5}.\]