your fixed expenses per month would be

Housing - 12000 (although if you look hard, you will get a 4 bedroom villa for 10K)
School fees - 7500
Phone/ Internet - 1000 (depending on your usage)

Adding up roughly it still gives you a balance of 19500 (40000-20500), which is approx $5300

So you have around $1000+ for your other expenses such as groceries, eating out, shopping, etc.

It's just a question of how flexible you are with the amount of $4000 that you wanna save, or should I say on the amount you wanna spend on miscellaneous stuff.