Explanation
From 2 to 198 there are 19 multiples of 10. These are 10, 20, 30.....180, 190.
This is an A.P series, whose sum is (n/2{a1 + aN}), where a1 and aN are the 1st and last terms respectively and total number of multiples are,
Total number ⇒ aN = a + (n-1)d
190 = 10 + (n-1)*10
180 = (n-1)*10
18 = n-1
n = 19
Now, a1 = 10, aN = 190
Sum ⇒ Sn = (19/2{10 + 190 }) = 1900
Average = Sum / Total number = 1900 / 19 = 100.