3D matric formula complete and how it is derived with all the working shown and a sample example solved

Jane is trying to decide how many tv shows she should watch tonight. she gets $10 of value from each tv show she watches. if she watches one show she gives up working on her math homework (which she values at $5). if she watches two, she also gives up her economics homework (which she values at $8). if she watches three then she also has to give up her biology homework (which she values at $12). using marginal analysis, how many shows does she watch? one