What is 15*9.

 Oct 10, 2017


 Oct 10, 2017

135, break it up

 Oct 10, 2017

This may help, if multiplying by 9 mentally is difficult. A lot of these mental tricks I wish I knew earlier on.


\(15*9\) Let's make it simpler. 
\(15(10-1)\) Obviously, this is the same thing. Now, distribute the 15 to both terms.
\(15*10-15*1\) Now, simplify. This is much easier, don't you think?
\(150-15\) And just in case borrowing is another mental headache...
\(150-(20-5)\) Yet again, I have not changed the expression at all. 
\(150-20+5\) Simplify.


This may look like a lot of steps, but all we are doing is making the problem easier. With practice, this will simplify multiplication with large numbers.

 Oct 10, 2017

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