Question

Twenty-five lamps are placed around a circle-shaped garden. How many straight lines could be found if each lamp is connected to all the rest ?

Answer 1

300

Explanation:

every lamp can connect other lamps ，from the first ，it can connect the other 24 lamps，and then， the second ，can also connect other 24 。but the line between the first and the second is counted twice。

so it would be：

（1+2+3+...+24）×24÷2=300