Economists are able to determine total utility by?

Total utility can be derived by adding marginal utility with the utility derived previously of the same good. For example, if the utility derived from the consumption of 3 apples is 20 utils and the marginal utility of consuming the 4th apple is 5 utils, then total utility derived of consuming 4 apples would be 25 utils (20+5). When marginal utility is positive for each successive quantity of good, the total utility increases. When marginal utility becomes zero, total utility stagnates. When marginal utility becomes negative, total utility starts to decrease.