My father plays an online game that requires him to do some calculations and find certain numbers by taking help from the previous records of the winning numbers and then book them within a limited time to win money.

He deposits some money to play the game. Once he has deposited the money, he does some mathematical calculations to find the possible winning number (he usually does this by analyzing the records of the previous winning numbers).
After he has found a series of possible winning numbers, he has them booked.

If the winning number is one of the numbers that he had booked, he gets some money as a reward. The bigger the sum you deposit to play the game, the bigger the sum you’ll win.

This is gambling.