A stack of books rests on a level perfectly smooth surface. A force F acts on the stack and it acclerated at 3m/s^2. A 1kg book is added to the stack. The same force is applied and now the stack accelerates at 2m/s^2. What was the mass of the original stack?

Question

A stack of books rests on a level perfectly smooth surface. A force F acts on the stack and it acclerated at 3m/s^2.  A 1kg book is added to the stack. The same force is applied and now the stack accelerates at 2m/s^2. What was the mass of the original stack?

I think i would use F=ma and F=m_total*a

I chose 5N to be the applied force. so F=ma 
5=m(3)
m=1.66

then F=m_total*a
5=(1.66+1)(2)
5=5.32
But this isn't valid. What am I doing wrong?