To Find: The value of x, i.e. the last term.
Given: The series and its sum.
The series can be written as x, (x + 3), …, 16, 19, 22, 25
Let there be n terms in the series
25 = x + (n – 1)3
3(n – 1) = 25 – xx = 25 – 3(n – 1) = 28 – 3n
Let S be the sum of the series

⇒ n[28 – 3n + 25] = 224
⇒ n(53 – 3n) = 224
⇒ 3n2 – 53n + 224 = 0

⇒ n = 7 as n cannot be a fraction.
Therefore, x = 28 – 3n = 28 – 3(7) = 28 – 21 = 7
The value of x is 7.