If a pyramid is made up of a stack of blocks in which each of the 10 layers...

Question

If a pyramid is made up of a stack of blocks in which each of the 10 layers in the stack has 10 fewer blocks in it than the layer under it, how many blocks are there in the pyramid if the top layer has 4 blocks?

Answer 1

This is an arithmetic series in which the first term is unknown (a₁), the last term is 4, there are 10 terms and the difference is -10.

The formula is 4 = a₁ + (10-1)(-9) so that a₁ = 94

The sum of the 10 terms of this sequence is (94 +4)(10/2)=490