Exercise 2.3.13 (Number of positive integers $\leq 25200$ prime to $3600$)

Proof. Since $k\wedge 3600=1$ is equivalent to $(3600+k)\wedge 3600=1$, there are as many integers prime with $3600$ in $[[1,3600]]$ than in $[[3601,7200]]$, and more generally than in $[[K\cdot 3600+1,(K+1)3600]]$ for all $K$.

Therefore, the number $N_7$ of positive integers $\le 25200$ that are prime to $3600$ is

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