91. A pasture is full of grass, which grows at a uniform rate every day. 17 cows can finish eating the grass in 30 days, 19 cows can finish in 20 days. Now, a certain number of cows eat for 5 days, then 3 cows are sold, and the remaining cows finish eating the grass in 2 more days. How many cows were there initially eating the grass (grass grows uniformly)?

原有 $\qquad$头牛吃草

Initially, there were $\qquad$ cows eating the grass.
Reference answer: 31