Arithmetic series Problem

In a certain sculpture, coils of wire are arranged in rows. The second row has two more coils than the first, the third two more than the second, and so on, to the tenth and final row. If there is a total of 120 coils of wire in the sculpture, how many coils are in the final row?

let first row = x 
second row = x+2 
third row = x+4 
... 
tenth row = x+18 

total = 10x + 2(1+2+3+...+9) 
total = 10x + 2*45 = 120 
x = 3 
thus, tenth row = x+18 = 21 

you can you use n(n+1)/ 2 for 1 to 9

Alternatively you can use formulas for arithmetic progression for this