What is the magnitude of acceleration in a system with two boxes and a pulley?

[madah12]

Homework Statement

http://img148.imageshack.us/img148/8686/ssslk.png
the two boxes and the pulley have the same mass
I=.5mr^2
the system is accelerating downwards calculate the magnitude of the acceleration

Homework Equations

The Attempt at a Solution

I will call the falling one D and the higher one U

mg-T_d=ma
T_d=mg-ma
T_u=ma
by torque
T_d-T_u=I alpha/r
T_d-T_u=I a/r^2
T_d-T_u=(.5mr^2) a/r^2
T_d-T_u==.5ma
mg-ma-ma=.5ma
2.5a=g
a=g/2.5=3.5
what's wrong here??
in the exam I got 4 out of 5 in this question I can't see why I didnt get a full mark I revised everything

 

[Beaker87]

a=g/2.5=3.5

Try doing this part again

Remember: g=9.81m/s²

 

[madah12]

uh lol so after all that work my error was numerical serves me right for using my head and not the calculator in this exam because I thought it was an easy division they should have only subtracted a half -.-