# Multiplication by 5

Often, instead of multiplying by 5, it is more convenient to multiply first by 10 and then divide the result by 2. For example,

137·5 = 1370/2 = 685.

Another examples:

342·5 = 3420/2 = 1710.

But how do you divide by 2 fast? One simple way is to split the number into parts so that each is divided by 2 easily. In the above cases:

 1370/2 = (1200 + 100 + 70)/2 = 600 + 50 + 35 = 685.

and also

 3420/2 = (3000 + 400 + 20)/2 = 1500 + 200 + 10 = 1710.

Note how useful here was the distributive law. • Multiplication by 9, 99, 999, (Multiply + Subtract) etc.
• Squaring 2-Digit Numbers
• Division by 5
• Multiplication by 2
• Multiplication by 9, 99, 999, etc. (Something Special)
• Product of 10a + b and 10a + c where b + c = 10
• Product of numbers close to 100
• Product of two one-digit numbers greater than 5
• Product of 2-digit numbers
• Squaring Numbers in Range 26-50
• Squaring Numbers in Range 51-100
• Squares of Numbers That End in 5
• Squares Can Be Computed Squentially
• How to Compute Fast Any Square
• Adding a Long List of Numbers
• 