Hi,

Answering your first question:

1. They are all good in the part of market-sizing questions
2. They are all pretty good in terms of quant problems as part of the case
3. They are all very bad in terms of the solutions and structures. Thus the casebooks solutions cannot be used as the best practices

Answering your second question, there are no mental math books. However, there are several math skills that you can develop:

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 lose zeros and all multiplications/divisions will be replaced with + or -.

In your case, it will look like 63 * 10ˆ5 / 72 * 10ˆ2

2) Learn how to multiply double digit numbers

Pls check this video - https://www.youtube.com/watch?v=6ndkkPZYJHo It takes an hour to learn this method

3) Learn how to divide the numbers fast

If you need the precise calculation - you can use the basic division technique, calculating 630 / 72 * 10ˆ2 = 875

If you don't need it precise - Learn the division table up to 1/11 (i.e. 5/6 = 83.3%). It will help you calculate any percentage problems

In your case, it will be roughly 6/7 * 10ˆ3 = 85.7 * 10ˆ3 or 857. Since I know that it was the rough calculation and the real number should be bigger, I would round the final result to 870

Best!