First, lets convert our dimensions into cm.
2*2.54 = 5.08 cm.
1.5*2.54 = 3.81 cm.
Now, lets multiply by for the get the dimensions of the original painting in inches.
5.08*40 = 203.2 cm
3.81*40 = 152.4 cm
Finally, lets divide our new dimensions.
203.2/2.54 = 80 in.
152.4/2.54 = 60 in.
The dimensions of the original painting is 80 in. by 60 in.