The venn diagram in the picture tells us if the students take either math or science or both or if they take neither. We must find the probability that a student is taking either math or science or both. Now let's see which students are included in this condition. First we have students that only take math, these are represented by the yellow area in teh diagram so we have 45. Then we have the students taking only science, they are represented by the blue region and they are 24. Finally, we have the students that take both which are represented by the green area and are 51. Then the number of students taking either math or science or both is 45+24+51=120. If we divide this number by the total number of students we find the probability we were looking for:
Then the answer is 0.795.