Explanation:
1)
the largest 5-digit number at all is
99,999
let's do a division by 145 :
99,999/145 = 689.6482759...
aha ! it fits 689 times fully into it. but not 670 times.
so 145×689 is the largest 5-digit number which is exactly divisible by 145.
145×689 = 99,905.
2)
you missed at least some brackets. and it looks like one "+" sign was turned into a "×" sign. and a "-" sign turned into a "+" sign. I can only assume what your original problem is. I hope you did not miss anything else. you really need to check what you write here.
I suspect you meant this :
[(-24) + (-13)] + (-27) = (-24) + [(-13) + (-27)]
[-24 - 13] - 27 = -24 + [-13 - 27]
-37 - 27 = -24 + [-40]
-64 = -24 - 40 = -64
both sides are equal, so it is correct.
3)
A = a + b + c
B = a - b + c
C = a + b - c
A + B - C = (a + b + c) + (a - b + c) - (a + b - c) =
= a + b + c + a - b + c - a - b + c =
= 2a - a + b - 2b + 3c =
= a - b + 3c
remember :
+ + = +
- - = +
+ - = -
- + = -