To count to 99, let the right fingers be worth 1 and the right thumb be worth 5. Let the left fingers be worth 10 each and the left thumb be worth 50. This makes it possible to represent any number from 0 to 99 on the hands. Start counting with both hands in fists to represent zero. Start extending the digits of the right hand one at a time to count from 1 to 9, going from 4 to 5 by tucking away the fingers and extending the thumb. Then put one left finger up and all the right hand into a fist, to represent 10. Continue this way until all digits are extended to represent 99.
To add two numbers together, begin with the first number on both hands. Add the second number on top of the first number. Sometimes this is easy, for example adding 13 to 15 requires representing 13 -- one left finger extended three right fingers extended -- and adding 15 more by extending the right thumb and one more left finger. Now there are two left fingers extended as well as three right fingers and the right thumb: This equals 28, the sum of 13 and 15. Sometimes, the process is more complicated, such as when you might have to carry a number or recognize that 3 fingers plus 3 fingers is a finger and a thumb.
To multiply a digit by nine, hold your hands before you with all fingers and thumbs extended and the backs of the hands facing you. To multiply 9 X 7, count over from the left -- starting with the left hand pinkie -- seven fingers until you get to your right hand index finger and bend it down. Read the answer by counting the fingers on either side of the bent finger: 63. To multiply 9 X 3. Hold up all the fingers, start counting with the left hand pinkie and count three fingers until you get to the middle finger of the left land and bend it down. count the fingers to ether side of the bent finger to get 27. Unfortunately, this only works for multiplying by 9.
Recall how many days are in each month by counting the knuckles and the depressions on your left fist. January is the first knuckle, representing a long month. February is the depression between the first two knuckles, representing a short month, March is the next knuckle, another long month. Continue until the last knuckle, which is July. Come back to the first knuckle, August, another long month and continue.