19x19 easy mental arithmetic

This is for learning and practicing mental arithmetic from 11x11 to 19x19 in a easy way.
If you are not doing mental arithmetic habitually, don't worry. This is an easy method of only 1 pattern, which you will get used to soon, and can make it fluently.
With only 3 problems to answer for 1 practice, you can launch it anytime.
It's not only for children and students, but also for businessmen or elderly persons for brain workout.