Since the two measures are vertical angles, you want to set them equal to each other :
6x+7=8x-17
-6x -6x
——————
7=2x-17
+17 +17
———————
24= 2x
X= 12
———
The answer is 12.
2 classes of 21 students each
21 × 2 classes = 42 students in total
42 × 3 mins = 126 minutes for 42 students
10 mins for 2 class pics = 20 minutes
126 + 20 = 146 minutes = 2 hours and 26 minutes
It should take 146 minutes (2 hours and 26 minutes) to take all the pictures.
