同学们好！今天我们要解决一个有趣的数学问题叫做牛吃草问题。想象一个神奇的牧场，草每天都在生长，同时牛在吃草。我们有两个情况：二十头牛可以在五天内吃完所有草，或者十五头牛可以在十天内吃完。我们的挑战是找出需要多少头牛才能在两天内吃完所有草。
要解决这个问题，我们需要理解牛吃草的特点。草地有两部分：原本就有的草，和每天新长出来的草。我们假设每头牛每天吃草量是一个单位，这样就能建立数学关系了。
现在让我们建立数学方程。设原草量为x，每天新长草量为y，每头牛每天吃草量为1单位。根据条件一，二十头牛五天吃完，总草量是原草量加五天新长的草，等于一百单位。根据条件二，十五头牛十天吃完，总草量是原草量加十天新长的草，等于一百五十单位。
现在来解这个方程组。我们用第二个方程减去第一个方程，得到五y等于五十，所以y等于十。然后把y等于十代入第一个方程，得到x加五十等于一百，所以x等于五十。答案是：原草量为五十单位，每天新长草量为十单位。
最后求解答案。要在两天内吃完，总草量等于原草量五十，加上两天新长的草二十，总共七十单位。如果需要在两天内吃完，牛数乘以二等于七十，所以牛数等于三十五。让我们验证一下：三十五头牛两天吃七十单位草，而草地总共有七十单位草，计算正确！
To solve this problem, we need to understand the characteristics of the cow eating grass problem. The grassland has two parts: the original grass that was already there, and the new grass that grows every day. We assume that each cow eats one unit of grass per day, so we can establish mathematical relationships.
Now let's establish mathematical equations. Let the original grass amount be x, and the daily new grass growth be y, with each cow eating one unit of grass per day. According to condition one, twenty cows finish in five days, so the total grass amount is original grass plus five days of new growth, equals one hundred units. According to condition two, fifteen cows finish in ten days, so the total grass amount is original grass plus ten days of new growth, equals one hundred fifty units.
Now let's solve this system of equations. We use the second equation minus the first equation to get five y equals fifty, so y equals ten. Then we substitute y equals ten into the first equation to get x plus fifty equals one hundred, so x equals fifty. The answer is: original grass amount is fifty units, and daily new grass growth is ten units.
Finally, let's solve for the answer. To finish in two days, the total grass amount equals the original grass amount of fifty, plus two days of new grass growth which is twenty, totaling seventy units. If we need to finish in two days, the number of cows times two equals seventy, so the number of cows equals thirty-five. Let's verify: thirty-five cows eating for two days consume seventy units of grass, and the grassland has exactly seventy units of grass total. The calculation is correct!