Carmen arranges stacks of blocks so that each successive level has 1 fewer block then the level below it and the top level has 1 block. Carmen wants to make such a stack with 12 levels. how many blocks does she need?

The top level will have 1 block, then the next level down will have 2, then the next level down will have 3, etc. If she wants 12 levels, then we’ll have to add a block 12 times!

1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 = 78