The level of arithmetic knowledge of a high school student was explored when solving additive word problems considering the semantic structure and syntactic component. The methodology was qualitative and developed in four stages: the first is the selection of the participant, the second is the design of a questionnaire with twenty additive problems, the third is the application of the semi-structured interview and questionnaire for the student to solve, and the fourth is the analysis of the data to identify the arithmetic knowledge. The results show that the student is at level 4 (directional relations level) solving comparison and equalization problems. However, he had difficulty in one of the comparison problems (incorrect application of the operation), but successfully solved the change, combination and equalization problems. It is concluded that the student can reinforce the comparison structure to have arithmetic knowledge of level 4 in its entirety.