Find the average of the first fifteen numbers multiples of 11.

Here the first number which is the multiple of 11 is 11. So the series will be 11, 22, 33, ...............

Now we need to obtain the 15th term of this series.

15th term of this series = $$first\ term+\left(number\ of\ terms-1\right)\times\ difference\ between\ consecutive\ terms$$

= $$11+\left(15-1\right)\times11$$

= $$11+14\times11$$

= 11+154

= 165

Average of the first fifteen numbers multiples of 11 = $$\frac{1st\ term+15th\ term}{2}$$

= $$\frac{11+165}{2}$$

= $$\frac{176}{2}$$

= 88