Goi so can tim la abcd (a_0)
a co 4 cach chon: 2,4,6,8 (a khac 0)
b co 5 cach chon: 0,2,4,6,8
c co 5 cach chon: 0,2,4,6,8
d co 5 cach chon: 0,2,4,6,8
Co tat ca so so co 4 chu so ma moi chu so dau la chan la:
4x5x5x5=500(so)
Dap so: 500so
có (9998 - 1000):1+1=8999 số chẵn