# Mental math method for large integer multiplication

I am practicing for trading interview, especially the quick calculation of mental math. But I am wondering is there any quick method to calculate the general multiplication? like the one -4.41 * 2.86. I know some methods for 5.01*6.99, as (5+0.01)(7-0.01). But how can I calculate a general multiplication of two simple integers or common numbers? Thanks!

• Most tricks are: factors, powers, sum (like you pointed out). Sometimes explicitly writing your equation like $ab=c$ and then do an operation on both sides that make the LHS look nicer can help. e.g. $0.36\cdot 0.25=p$ then $\sqrt{0.36\cdot 0.25}=\sqrt{p}$ then $0.6\cdot 0.5= \sqrt{p}$, which is clearly $0.3=\sqrt{p}$, thus $p=0.09$. That example can obviously be done with just factorising with powers of 10, but it lets you turn some number with an annoying decimal into something usable. You’re basically turning the problem into a “sub-problem” But those are pretty much the main tricks. Jan 27 at 9:56
• Thanks @THAT'SMYQUANTMYQUANTITATIV. Thanks for sharing the tips about the square. For this case (posted in the question), yes it can be factorised with powers of 10. But how can we do the following larger number integer calculation? i.e. 441*286? Jan 27 at 10:28

$$-4.41*2.86 = -(440*286+286)/10,000=-(400*286+40*286+286)/10,000=-(114,400+11,440+286)/10,000=-12.6216$$