Example by my side:
36 + 45 - 45 + 33 = ?
you don't need to wasting time to count 45 - 45
Throw it away, then you just need to counting 36 + 33 = 69
it's make simply.
other example =
47 + 32 - 33 + 45 = ?
counting the midle number first, 32 - 33 = -1
then, 47 + 45 = 92
then 92 + (-1) = 91.
but, it's depends the number of maths to count,
how many other players can solving faster than you.