how is total utility derived from marginal utility?

Total Utility means the Utility-in-total YET derived. Hence to get total utility yet derived from consumption of good-X, first select the number of units consumed for which you want to find yet Total Utility derived (suppose it be 5 units). Now add all the Marginal Utilities in the schedule written for units consumption from 0 unit to 5 units of good-X. Here is your TU at 5 units of good-X !!