Concept:1). Sum of n terms in AP is given by Sn = \(\frac{n}{2}\)[2a + (n ? 1) × d]2). For a given set of AP, the Arithmetic mean is equal to its median.Arithmetic mean = median = \(\frac{1}{2}\)(a + l)Where a = First term, l = last term.Calculation:Sum of n terms in AP is given by Sn = \(\frac{n}{2}\)[2a + (n ? 1) × d]? Sn = \(\frac{51}{2}\)[2 × 4 + (51 ? 1) × 3]? Sn = 4029\(\displaystyle Mean = \frac{Sum \ of\ all \ the\ observations}{Total\ number \ of \ observations}\)? Mean = \(\displaystyle \frac{4029}{51}\)? Mean = 79The arithmetic mean of the given marks is 79,Which is also equal to the median of the given AP.? The median of the marks is 79.