Not 100% sure if I'm correct but if Debra's garden is 12 feet in length and 10 feet in width then you would multiply 10 × 12 that would be 120. Then divide 120 to 30 and you get your answer, 4. ( not 100% sure )
First find the area of the rectangle by multiplying 10 and 12 ft together. You should get 120 sq ft. Then divide by the bags that cover 30 sq feet and you get 40. 40 bags are needed :)