Maths

Multiplication by 5
It's often more convenient instead of multiplying by 5 to multiply first by 10 and then divide by 2.
For example, 137·5=1370/2=685.

Division by 5
Similarly, it's often more convenient instead to multiply first by 2 and then divide by 10.
For example, 1375/5=2750/10=275.

Division/multiplication by 4
Replace either with a repeated operation by 2.
For example, 124/4=62/2=31. Also, 124·4=248·2=496.

Division/multiplication by 25
Use operations with 4 instead.
For example, 37·25=3700/4=1850/2=925.

Division/multiplication by 8
Replace either with a repeated operation by 2.
For example, 124·8=248·4=496·2=992.

Division/multiplication by 125
Use operations with 8 instead.
For example, 37·125=37000/8=18500/4=9250/2=4625.

Squaring two digit numbers.
You should memorize the first 25 squares:

Squares of numbers from 26 through 50.
Let A be such a number. Subtract 25 from A to get x. Subtract x from 25 to get, say, a. Then A²=a²+100x.
For example, if A=26, then x=1 and a=24. Hence 262=242+100=676. Similarly, if A=37, then x=37-25=12, and a=25-12=13. Therefore, 372=132+100·12=1200+169=1369. Why does this work? (25+x)2-(25-x)2=·=50·2x=100x.

Squares of numbers from 51 through 99.
The idea is the same as above. (50+x)2-(50-x)2=100·2x=200x.
For example, 632=372+200·13 = 1369+2600 = 3969.

Squares of numbers from 51 through 99, second approach.
We are looking to compute A2, where A=50+a. Instead compute 100·(25+a) and add a2. Example: 572. a=57-50=7. 25+7=32. Append 49=72. Answer: 572=3249.

In general, a2 = (a + b)(a - b) + b2. Let a be 57 and, again, we wish to compute 572. Let b = 3. Then 572 = (57 + 3)(57 - 3) + 32, or 572 = 60·54 + 9 = 3240 + 9 = 3249.

Squares of numbers that end with 5.
Let A=10a+5. Then A2=(10a+5)2=100a2+2·10a·5+25=100a(a+1)+25.
For example, to compute 1152, where a=11, first compute 11·(11+1)=11·12=132 (since 3=1+2). Next, append 25 to the right of 132 to get 13225! Another example, to compute 2452, let a=24. Then 24·(24+1)=242+24=576+24=600. Therefore 2452=60025. Here is another way to compute 24·25: 24·25=2400/4=1200/2=600. The rule naturally applies to 2-digit numbers as well. 752=5625 (since 7·8=56).

Product of two one-digit numbers greater than 5.
This is a rule that helps remember a big part of the multiplication table. Assume you forgot the product 7·9. Do this. First find the access of each of the multiples over 5: it's 2 for 7 (7 - 5 = 2) and 4 for 9 (9 - 5 = 4). Add them up to get 6 = 2 + 4. Now find the complements of these two numbers to 5: it's 3 for 2 (5 - 2 = 3) and 1 for 4 (5 - 4 = 1). Remember their product 3 = 3·1. Lastly, combine thus obtained two numbers (6 and 3) as 63 = 6·10 + 3.

The explanation comes from the following formula:
(5 + a)(5 + b) = 10(a + b) + (5 - a)(5 - b)
In our example, a = 2 and b = 4.

Product of two 2-digit numbers.
If the numbers are not too far apart, and their difference is even, one might use the well known formula (a+n)(a-n)=a2-n2. a here is the average of the two numbers.
For example, 28·24=262-22=676-4=672 since 26=(24+28)/2. Also, 19·31=252-62=625-36=589 since 25=(19+31)/2.

If the difference is odd use either n(m+1)=nm+n or n(m-1)=nm-n.
For example, 7·34=37·35-37=362-12-37=1296-1-37=1258. On the other hand, 37·34=37·33+37=352-22+37=1225-4+37=1258.

Product of numbers that only differ in units.
If the numbers only differ in units and the sum of the units is 10, like with 53 and 57 or 122 and 128, then think of them as, say 10a+b and 10a+c, where b+c=10. The product (10a+b)(10a+c) is given by 100a2+10a(b+c)+bc =&nbs;100a(a+1)+bc. Thus to compute 53 times 57 (a=5, b=3, c=7), multiply 5 times (5+1) to get 30. Append to the result (30) the product of the units (3·7=21) to obtain 3021. Similarly 122·128 = 12·13·100+2·8=15616.

Multiplying by 11.
To multiply a 2-digit number by 11, take the sum of its digits. If it's a single digit number, just write it between the two digits. If the sum is 10 or more, do not forget to carry 1 over.
For example, 34·11=374 since 3+4=7. 47·11=517 since 4+7=11.

Faster subtraction.
Subtraction is often faster in two steps instead of one.
For example, 427-38=(427-27)-(38-27)=400-11=389. A generic advice might be given as "First remove what's easy, next whatever remains". Another example: 1049-187=1000-(187-49)=900-38=862.

Faster addition.
Addition is often faster in two steps instead of one.
For example, 487+38=(487+13)+(38-13)=500+25=525. A generic advice might be given as "First add what's easy, next whatever remains". Another example: 1049+187=1100+(187-51)=1200+36=1236.

Faster addition, #2.
It's often faster to add a digit at a time starting with higher digits.
For example, 583+645=583+600+40+5=1183+40+5=1223+5=1228.

Multipliply, then subtract.
When multiplying by 9, multiply by 10 instead, and then subtract the other number.
For example, 23·9=230-23=207. The same applies to other numbers near those for which multiplication is simplified. 23·51=23·50+23=2300/2+23=1150+23=1173. 87·48=87·50-87·2=8700/2-160-14=4350-160-14=4190-14=4176.