Quickly Calculate Without a Calculator

Calculating Math Inside you Head

The Teen Times

example:
18
x17

Take the biggest number (18) and add the ones column of the smaller number (7)

18+7=25

Now add a zero at the end (250)

Now add 8×7 which = (56)

Add the 2 numbers together

250
+56
306

That’s your answer. This works only if both numbers are between 10-19

Other examples:

12×12=140+4=144
15×12=170+10=180
19×19=280+81=361

The Early Hundreds
Works if multiplying two numbers between 101-109

example:
104
x109

drop the 1 from the hundreds column (1) add the ones columsn (4+9=13) and times the one column (4×9=36)

Now put it together (1),(13),(36)
The answer is then: 11336

By the way this will work for numbers over 109, just be sure to carry the hundreds

example:

112
x120

(1) same 12+20=(32) same 12*20=240… keep the 40 add the 2 in the hundred column to the 32 to make it (34)
Now put it together (1),(34),(40)

13440

Squaring by 5’s

example:
65
x65

take the number in the tens column (6) and add 1 to it (7)
6×7=(42) The last two numbers are always (25)
Now put it together (42),(25)

The answer is then: 4225

Other examples:

85×85=(9×7=63)&25=6325
125×125=(12×13=156)&25=15625

Almost 100

example:
91
x95

91 is 9 away from 100, 95 is 5 away

91 (-9)
x95 (-5)

Add -9 to the bottom number (95-9)
or
Add -5 to the top number (91-5)

either way you get 86 – this is you thousands and tens column

now times the two derived numbers together (-9*-5)=(45)

Put it together (86),(45) and you get 8645

Other examples:

95 (-5)
x95 (-5)

From the square rule we already know that the answer is 9×10 & 25 or 9025 so we can easily check our answer

95-5=(90) and -5x-5=(25) so it’s 9025

91 (-9)
x94 (-6)

91-6=(85) & 9×6=(54) – 8554

81 (-19)
x94 (-6)

94-19=(75) & 19×6=(114) (make sure to carry the 1) – 7614

Almost 200
Same as Almost 100 but double the number you get for the thousands/hundreds column

example:
196 (-4)
x198 (-2)

198-4=194×2=(388) & -2x-4=(8) – 38808