Here's the answer -

number all the 6 surfaces from first to last 

charge on 1 = -q/2

2 = 3q/2

3 = -3q/2

4 = +3q/2

5 = -3q/2

6 = -q/2

 

 

just remember this = If there are n plates and charges given on each are q1,q2,q3,q4,... qn . Therefore , charge on outer surfaces of first and last plate is   (q1+q2+q3+q4+q5+.......qn)/2