We call a number a descending number if each digit is strictly smaller than the digit that comes before it. For example, 863 is a descending number. How many 4-digit descending numbers are there?
3210 4210 4310 4320 4321 5210 5310 5320 5321 5410 5420 5421 5430 5431 5432 6210 6310 6320 6321 6410 6420 6421 6430 6431 6432 6510 6520 6521 6530 6531 6532 6540 6541 6542 6543 7210 7310 7320 7321 7410 7420 7421 7430 7431 7432 7510 7520 7521 7530 7531 7532 7540 7541 7542 7543 7610 7620 7621 7630 7631 7632 7640 7641 7642 7643 7650 7651 7652 7653 7654 8210 8310 8320 8321 8410 8420 8421 8430 8431 8432 8510 8520 8521 8530 8531 8532 8540 8541 8542 8543 8610 8620 8621 8630 8631 8632 8640 8641 8642 8643 8650 8651 8652 8653 8654 8710 8720 8721 8730 8731 8732 8740 8741 8742 8743 8750 8751 8752 8753 8754 8760 8761 8762 8763 8764 8765 9210 9310 9320 9321 9410 9420 9421 9430 9431 9432 9510 9520 9521 9530 9531 9532 9540 9541 9542 9543 9610 9620 9621 9630 9631 9632 9640 9641 9642 9643 9650 9651 9652 9653 9654 9710 9720 9721 9730 9731 9732 9740 9741 9742 9743 9750 9751 9752 9753 9754 9760 9761 9762 9763 9764 9765 9810 9820 9821 9830 9831 9832 9840 9841 9842 9843 9850 9851 9852 9853 9854 9860 9861 9862 9863 9864 9865 9870 9871 9872 9873 9874 9875 9876 Total = 210 such numbers