How would you calculate this exercise in your head?
23 **·** 4 = ?
13 **·** 5 = ?
Here are a few suggestions about how you can think when you solve exercises. Think first about how you would do the calculation.
Example 1:
23** ·** 4 = ?
A. Multiply the tens and ones separately.
23 **·** 4 = (20 **·** 4) + (3 **·** 4) = 80 + 12 = 92
B. Increase and subtract.
23 **·** 4 = (25 **·** 4) – (2 **·** 4) = 100 – 8 = 92
C. Double, double.
23 **·** 4 = 23 **·** 2 **·** 2 = 46 **·** 2
= 92.
Example 2:
13 **·** 5 = ?
A. Multiply the tens and the ones for themselves.
13 **·** 5 = (10 **·** 5) + (3 **·** 5) = 50 + 15 = 65
B. Multiply by 10 and halve (divide by 2).
13 **·** 5
= (13 **·** 10)/2 = 130/2 = 65 |