Derivatives

#1
I would like to know the following things only:
1) how do you earn profit in F & O market, plz provide examples?
2) suppose i buy a futures contract worth rs. 1lakh pay for 2 mths futures and pay initial margin of rs. 20k will i have to pay the remaining 80k at expiry and say the share price is rs. 100 when i bought it how will i earn a profit?

thank you for helping
 

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