arithmetic sequence

Question

A collection of building blocks is set up into 51 rows. The first row has 124 blocks, and each row after that one has 4 more blocks than the preceding row. How many total blocks are there in the structure?

Answer 1

124, 128, 132, 136, ....an...320, 324

d=4 = common difference

an = a1 + d(n-1)

a51 = 124 + 4(50) = 124+200 = 324

sn = n(a1+an)/2

= (51/2)(124 +324) = 25.5(448) = 11,424