OK, it's a while since I've done any of this so double check it just in case, but here's what I think:-
With smaller mass , t=2pi*root(m/k) . So, rearrange this to find k=(4pi*pi*m)/(t*t)
Now, let new mass = M and new period = T. So, New period T = 2pi*root(M/k).
Rearrange this to show M=(k*T*T)/(4pi*pi).
Now substitute the equation above for k to get M=(m*T*T)/(t*t) and substitute real values for m=0.5, T=2, and t=1.5 to get M= 0.889kg.
So increase in mass is 0.389kg.