[tex]\displaystyle 60+(3+6+9+...+900):(6000:1000)= \\ \\ =60+3(1+2+3+...+300):6=60+3 \cdot \frac{300(300+1)}{2} :6= \\ \\ =60+ 3\cdot \frac{300 \cdot 301}{2} :6=60+3 \cdot \frac{90300}{2} :6=60+3 \cdot 45150:6= \\ \\ =60+3 \cdot 7525=60+22575=22635[/tex]
