Question

Second and fourth terms of the following arithmetic sequence are missing. Find the numbers at these positions.
$11$, ____, $19$, _____, _____

Answer 1

First term $=11$
Third term $=19$
Let the second term be $x$ and the common difference be $d$.
Then, $x-11=19-x$
i.e. $2x=30$
i.e. $x=15$
Thus, $d=15-11=4$
Hence, the fourth term will be $19+4=23$