This is a problem from the second pattern recognition video on Prep Swift. Since we are to add digits, I thought -2 should have been added here too as it is the first ‘term’. Is that correct? If not, why? I thought digits could be negative…
Here, -2, 88, 988, 9988, etc. are just to show a pattern. We only require to add digits of 10^30-12.
In this problem, we are asked to find the sum of all the digits of 10^30-12. So, we only need to focus on the resultant digits of 10^30-12. With the above pattern, we can easily recognize what would be the result of 10^30-12 . Now, look carefully at 10^3-12, it has one 9 and two 8’s: and subsequent results have the same number of 8’s but the number of 9’s increases with one increment. So, from 10^3 to 10^30 , there will be 28 numbers of 9’s and 2 8’s. Simply, do multiplication 28x9 + 2x8= 268.