So,
The area of art can be found multiplying:
8in * 10in = 80in²
And, the area of the frame, can be also found multiplying the dimentions:
4in * 6in = 24in².
If we divide, we'll obtain a ratio between the area of the frame and the area of the art as follows:

And, 0.3*100% = 30%.
So,30 percent of the original sketch was still able to be included in the frame.