You write an equation having 2 fractions on either side
It would look like this general template
A/B = C/D
So for your example
A = number of pages in case 1
B = number of minutes in case 1
C = number of pages in case 2
D = number of minutes in case 2
Meaning that..
A = 6
B = 2
C = x
D = 5
Leading to this equation
6/2 = x/5