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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!

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  • $\begingroup$ 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. $\endgroup$ Jan 27 at 9:56
  • $\begingroup$ 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? $\endgroup$
    – Xu Shan
    Jan 27 at 10:28

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I would calculate it as:

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

But this definitely is not the best way. I expended quite abit of brainpower doing this the mental way. Had some careless mistakes too before double checking as well.

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  • $\begingroup$ Thanks for sharing your method! How long did you take to break it down and get the final results? I found I was super slow on this kind of multipilcation... $\endgroup$
    – Xu Shan
    Jan 27 at 10:29
  • $\begingroup$ About 10 minutes? $\endgroup$
    – KaiSqDist
    Jan 27 at 10:30
  • $\begingroup$ Hi: I don't know if this is faster for you: you could do 4 * 286 and add 4 zeros. that's 114400. then you could do 41 * 200 = 8200. then you could 41 * 86 = 3526. the whole things adds up to 126126. I tend to use that approach but everyone has different taste. It took me about 5 minutes but I kept getting a different answer from above so maybe it could have been a little faster. $\endgroup$
    – mark leeds
    Jan 27 at 11:24

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