Interesting Maths Calculation 1

Let's do some maths.

67 x 63 = ????

I give you 3 seconds do to it.

I guess you may think,
'3 seconds? That's crazy!'

Are you?

Let me show you a fast way!

67 x 63

We will take the 6 and plus 1.
Then, we use it to time with 6.
(6+1) x 6 = 42

Then,
we will use the number behind to multiple together.
7 x 3 = 21

Lastly,
combine the number: 4221

Am I correct?
Check out with your calculator!

Let's try another example.
72 x 78 = ????

(7+1) x 7 = 56
2 x 8 = 16
Answer: 5616

I am correct again,
right?

However,
there are two conditions to have this method works.

1. The front numbers for both numbers have to be same.
67 x 63-> Both are 6
72 x 78-> Both are 7

2. The sum of last number for both numbers have to be 10.
67 x 63-> 7 + 3 = 10
72 x 78-> 2 + 8 = 10

Let's try another one.

131 x 139

Condition 1: Both front numbers are 13 (Passed)
Condition 2: 1 + 9 = 10 (Passed)

(13+1) x 13 = 182
1 x 9 = 9
Answer: 18209

Correct?

Cool?

Interesting?