How many terms are added in 24 + 20 + 16 + ......... to make the sum 72 ?

Sum of n terms =Sn=72⇒n2[2a+(n−1)d]=72⇒n{48+(n−1)(−4)}=144⇒n(−4n+52)=144      where a= first term =24 , d= common difference = 20-24=-4 ⇒n(n−13)=−36⇒n2−13n+36=0⇒n=4 (or)  9