7.2. Let $a$ be the number of six-digit numbers divisible by 13 but not divisible by 17, and $b$ be the number of six-digit numbers divisible by 17 but not divisible by 13.

Find $a-b$.
Answer: 16290. Solution. Let $c$ be the number of six-digit numbers divisible by both 13 and 17. Then $a+c$ is the number of all six-digit numbers divisible by 13, and therefore $a+c=\left[\frac{999999}{13}\right]-\left[\frac{99999}{13}\right]=76923-7692=69231$.

denotes the integer part of the number $x$ ). Similarly,

$b+c=\left[\frac{999999}{17}\right]-\left[\frac{99999}{17}\right]=58823-5882=52941$. Thus, $a-b=(a+c)-(b+c)=$ $=16290$.