These number exercises will help improve your concentration and mental endurance.
1. Recite all the numbers between 1 and 100 that contain the digit 3 (3, 13, 23...).
2. Count down from 200 by 7s (200, 193, 186...).
3. Recite the numbers by alternatly counting up by 2s and by 3s (2-3, 4-6, 6-9, 8-12...).
4. Try reciting a number series while writing down a different series. Pick any number series you want (up by 2s, down by 7s, every number containing 4s, etc.).
Feel free to modify these exercises and practice them to increase your ability.