All numbers divisible by 9 are divisible by 3, so you want all multiples of 9 that are 5 digit numbers.
10000 ÷ 9 = 11111/9 → first multiple of 9 that is a 5 digit number is 1112 x 9 = 10008
100000÷ 9 = 111111/9 → last multiple of 9 that is a 5 digit number is 11111 x 9 = 99999
→ there are 11111 - 1112 + 1 = 10,000 five digit numbers that are divisible by 9 (and 3). I'll let you work some out (hint: multiple any number between 1112 and 11111 inclusive by 9).
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