Tu 1 den 9 can 9 so co1 chu so vay can 9 chu so
Tu 10 den 99 can 90 so co2 chu so vay can 180 chu so
Tu 100 den 999 can 990 so co3 chu so vay can 2970 chu so
Tu 1000 den 2016 can 1017 so co4 chu so vay can 4068 chu so. vay can tat ca 9+180+2970+4068=7236chu so