RRB RPF RPSF SI and CONS Model Papers 7 with solutions

(2) 22 + 24 + 26 + ... + 50
= 2 (11 + 12 + 13 + .... + 25)
= 2 [(1 + 2 + 3 + ... + 25) – (1 + 2 + 3 ... + 10)]
$=2\left(\frac{25\times 26}{2}-\frac{10\times 11}{2}\right)$
= 2 (325 – 55) = 2 × 270 = 540
Tricky Approach
Sum of first n even numbers = n (n + 1)
∴ Required sum = Sum of 25 even numbers from 1 to 50 – sum of 10 even numbers from 1 to 20
= 25 × 26 – 10 × 11 = 650 – 110 = 540