Bunuel wrote:
In sequence T, each term after the first term is d more than the preceding term. The sum of the first 10 terms of T is 210. The sum of the first 20 terms of T is 820. What is the value of d?
A. 4
B. 4.5
C. 5
D. 5.5
E. 6
The common difference indicates that the numbers are in arithmetic progression.
Let the first term be x; common difference d
Sum of n terms in AP (or equally spaced set) = Middle Term * Number of terms
Sum of first 10 terms = 210
210 = 10 * ( \(\frac{x+4d + x +5d}{2}\))
210 = 10 * ( \(x + 4.5 d\))
21 = x + 4.5d ------(1)
Sum of first 20 terms = 820
820 = 20 * (x + 9.5d)
x + 9.5d = 41 ------(2)
Subtracting equation (1) from equation (2)
20 = 5d
d = 4
Option A
Statistics : Posted by gmatophobia • on 23 Nov 2022, 08:30 • Replies 3 • Views 87
