It is important to be able to do mental calculations and apply different strategies for calculating. Mental calculations can make it easier to find the product without using a calculator or pencil & paper.
One method that can be used is the Compatible Numbers property.
For example: 4x2x25x5 - this problem looks fairly difficult but it can be easy if we use the method.
1st, find numbers that are easy to multiply together such as 4x25, and 2x5. Since we know 4x25 is 100 and 2x5 is 10, we are left with 100x10
2nd, multiply the remaining numbers. 100x10=1000
Subtraction/Distribution property
ex: 96x30
1) 100 is an easier number to work with than 96, so we can write 96 as 100-4.
2) We are left with (100-4)x30 which is the same as 30(100-4)
3) Distribute the numbers: 30x100= 3000 and 30 x-4 = -120
4) We are left with 3000-120 which equals 2880.
These are the mental calculations I found the easiest to use and the most helpful. However, there are a lot more methods and this website describes many more of them. http://www.cut-the-knot.org/arithmetic/rapid/rapid.shtml
Also, i've included a video that I found very interesting. It teaches a method of multiplying that uses the base itself (base 10) to mentally calculate the product.
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