Multiply any two digits numbers with the same first digit and the second digit that sums up to 10

Multiply any two digits numbers with the same first digit and the second digit that sums up to 10
Let’s say that you want to multiply 42 and 48 together. Notice that they both start with 4, and that the sum of their second digit is 10. In this case there’s a simple rule that you can use to find their product. Simply multiply the first digit (4) for the nexthigher number (5) and then append the product of their second digits.

For example
1).42*48
now,
(4*5)(2*8)=2016
2).91*99
Now,
(9*10)(1*9)=9009

Note that if the product of the second digits is below ten, you have to add a 0 in front of it.

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