I am a regular polygon with a curious property: the number of internal diagonals I contain is exactly equal to my number of sides. How many sides do I have?
5
The number of diagonals in an n-sided polygon is given by n(n - 3) / 2. Setting this equal to n yields n(n - 3) / 2 = n. Dividing both sides by n gives (n - 3) / 2 = 1, so n - 3 = 2, meaning n = 5 (a pentagon).
About this riddle
Math riddles hide a calculation inside a short story. Pull out the numbers, work out what is being asked, and watch for a trick such as a unit change.
Difficulty: Medium. More like this: Math riddles with answers or try Target 24 for a different kind of puzzle.
More math riddles
There are 14 birds in a box and 4 are taken out. How many birds are left?
Omar makes 5 rows of 2 rainbows and adds 6 more at the end. How many rainbows are there in all?
Eli gets 7 clocks on Monday and 8 clocks on Tuesday, then gives away 6. How many clocks are left?
A mother is currently 4 times as old as her daughter. In 20 years, the mother will be twice as old as her daughter. How old is the daughter right now?
There are 15 bunnies in a box and 6 are taken out. How many bunnies are left?
I am a two-digit prime number under 50. If you reverse my digits, you get another prime number that is 36 larger than I am. What number am I?