20C15{ }^{20} \mathrm{C}_{15{}}20C15
20C10{ }^{20} \mathrm{C}_{10}20C10
20C5{ }^{20} \mathrm{C}_{5}20C5
20C15×20C5{ }^{20} \mathrm{C}_{15} \times{ }^{20} \mathrm{C}_{5}20C15×20C5