Question

Take 9 from 6, 10 from 9, 50 from 40 and leave 6.

Answer 1

Done.
Here is the 6 that I left ==> 6

Answer 2

SIX - 9 (IX) = S

9 (IX) - 10 (X) = I

40 (XL) - 50 (L) = X

SIX

Explanation:

its the roman numbers

so SIX-IX= u stay with the x and so on

hope this helped :D