how many terms of the AP24,20,16 must be taken so that the sum maybe 72?

             first term =24 , common difference = --4

    Sn =  n / 2 {  2a + (n-1)d }   = 72

               n / 2 {  2 x24 + (n - 1) x-4}  = 72

            n { 48  -4n +4 }   = 72x2 = 144

           n(52 - 4n }  = 144 ===>  4n^2 - 52n  + 144 = 0

           n^2 - 13n +36 = 0 ===> n = -9 , -4   { number of terms  should be positive  so  n = 9  , 4}

                   no of terms  is equal to 9   & 4