Dear Nalini, 

There must be either a total column given besides the individual marks of practical and theory; or there must be some percentage suggested by your examination board such that Practical marks will contribute let's say 30% towards final marks, and theory marks will contribute 70% marks. So as per our example, if you scored 100/100 in theory, 70% of it would be contributed towards final marks, which means it will give 70 marks, and if you scored 50/100 in practical, 30% of it would be contributed, which means it will give 15 marks. So, 70+15, your total final marks would become 85 of 100. 

Hope it clarifies. 
Good Luck!