90C10=90C80=90!/(90-10)!10!=90!/80!10!=90×89×...×81/10!.
90.89.88.87.86.85.84.83.82.81/10.9.8.7.6.5.4.3.2.1=
89.11.29.43.17.83.41.81=1220813×4685931=5716835820000+3809661903=5,720,645,481,903.
Note: 9×10=90; 8×11=88; 3×29=87; (2×3=6); 2×43=86; 5×17=85; 7×4×3=84; 2×41=82.