A group of children painted letters of the alphabet on some cube bricks. They did it in a special way. They painted A on one brick, B on two bricks, C on three bricks, and so on.



How many bricks had they painted by the time they had done the Cs?

How many bricks had they painted by the time they had done the Ds?

How many bricks had they painted by the time they had done the Es? And the Fs?

And the Gs?

And the Hs?

And the Is?

And the Js?

...

...?

Can you see why these totals are called triangle numbers?