How I would do the examples you provided:

24*45 = 10*45*2 + 4*45 = 900+180=1080

63*18 = 63*20-(63*2)=1260-126=1,134

91*22=90*22 +1*22 =90*20+90*2 +22 = 1800+180+22=2,002

In general, I try break up numbers into smaller segments that are easier to manage. It also depends if it's easier to subtract (as I did in the second example above) or just add up the various segments (as I did in example 1).

In general, just a lot of practice with this will get you a lot better and enable you to solve questions like this in your head in a short amount of time - but I would still write it out in an interview to be 100% sure!

Hi,

I use the following method and recommend it to all of my students:

https://www.youtube.com/watch?v=6ndkkPZYJHo

You can learn to do it on paper fast in 1-2 hours, and it will take couple of days to train doing it mentally.

Other useful tools:

1) Learn how to work with zeros. Best way - always use 10^power instead of zeros

Example:

300x9000 = 3*10ˆ2 x 9*10ˆ3=3x9*10ˆ(2+3)=27*10ˆ5

Handwritten it looks not that complicated. If you get used to writing all the numbers that way, you will never loose zeros and all multiplications/divisions will be replaced with + or -.

2) Learn the division table up to 1/11 (i.e. 5/6 = 83.3%). It will help you calculate any percentage problems

Best

(editiert)

Hi Anonym,

you may have a look at this video:

https://www.youtube.com/watch?v=6ndkkPZYJHo

best regards