The numbers lying between 300 and 700 which are multiples of 9 are
306, 315, 324,…, 693
a2 – a1 = 315 – 306 = 9
a3 – a2 = 324 – 315 = 9
∵ a3 – a2 = a2 – a1 = 9
Therefore, the series is in AP
Here, a = 306, d = 9 and an = 693
We know that,
an = a + (n – 1)d
⇒ 693 = 306 + (n – 1)9
⇒ 693 - 306 = (n – 1)9
⇒ 387 = (n – 1)9
⇒ 43 = (n – 1)
⇒ n = 44
Now, we have to find the sum of this AP
⇒ S44 = 22[612 + 387]
⇒ S44 = 22[999]
⇒ S44 = 21978
Hence, the sum of all numbers lying between 300 and 700 is 21978.