From Wikimedia user Cmglee, two digital arithmetic techniques:

Above: To multiply a positive single-digit integer by 9, hold up your hands palm up, imagine the fingers numbered consecutively 1 to 10, and fold down the finger corresponding to the number to be multiplied (here, 8). The product is the two-digit number represented by the remaining two groups of fingers — here there are seven fingers to the left of the folded finger and 2 to the right, so 9 × 8 = 72.

Below: To multiply two integers between 6 and 10, imagine each hand’s fingers numbered from 6 (pinky) to 10 (thumb), as shown. Fold down the two fingers corresponding to the factors, as well as all fingers between these two (in this example we’ll calculate 6 × 7, so fold down finger 7 on the left hand, finger 6 on the right, and the finger that lies between them, the left pinky). Count the remaining upraised fingers on the left hand (3), multiply that by the remaining upraised fingers on the right hand (4), and add 10 times the number of folded fingers (30). 3 × 4 + 30 = 42.