We know that
• There are 13 students in group A.
,
• 25% of the class is in group A.
,
• 75% of the class is in group B.
Based on the given information, 13 students represent 25% of the class, which is group A. To find the numbers of students in group B, we can form the following proportion.

Then, we solve for x.

Hence, there are 39 students in group B.