Without using a calculator, paper, etc. do the following counting practices. During each practice you should not be disturbed or interrupted; otherwise restart it. Remember that this is a great exercise for you to improve your concentration helping your memory. After each practice you may rest for 1 - 3 minutes and not more.
1) Count from 11 to 91 like this (add 8 to a number to get the next number):
11, 19, 27, 35, ..., 91
2) Count from 81 to 1 like this (subtract 4 from a number to get the next number):
81, 77, 73, 69, ..., 1
3) Count from 111 to 231 like this (add 6 to a number to get the next number):
111, 117, 123, ..., 231
4) Count from 181 to 91 like this (subtract 3 from a number to get the next number):
181, 178, 175, ..., 91
