3. In Grade 5, there are Class A and Class B. The number of students in Class A is $5 / 7$ of the number of students in Class B. If 3 students are transferred from Class B to Class A, the number of students in Class A will be 4/5 of the number of students in Class B. Class A originally had $\qquad$ students.
【Analysis】The original ratio of the number of people was 甲: $乙=5: 7=15: 21$, after the adjustment, the ratio became 甲: $乙=4: 5=16: 20$. The total number of people remained unchanged before and after the adjustment, so the total number of people is $[(5+7),(4+5)]=36$ parts. The adjustment in the ratio shows a change of 1 part, therefore, 1 part is 3 people, so the original number of students in Class 甲 is $15 \times 3=45$ people.