我们需要找到一种方法，用数字3、7、5、4通过基本数学运算来得到21。让我们考虑不同的组合，看看哪种方法有效。
让我们先尝试一些基本的组合。简单相加得到19，不够。试试乘法：3乘以7等于21，再减5加4得到20，还是不对。用括号改变运算顺序：3加7等于10，乘以5得到50，除以4得到12.5，太小了。再试试3乘以7加5的和，等于36，减去4得到32，太大了。
让我试试这个组合：7减4等于3，5加3等于8，3乘以8等于24。咦，这是24，不是21！让我重新思考。其实最简单的方法是直接用3乘以7等于21，这样我们只用了两个数字，还剩下5和4没用到。
经过多次尝试，我找到了一个可行的解法：5加4等于9，乘以3得到27，减去7得到20，再加1得到21。等等，这里用了额外的1，不符合规则。让我重新思考这个问题，看看是否真的有解。
Now let's solve this step by step. First, we calculate 7 times 3, which equals 21. Next, we calculate 5 minus 4, which equals 1. Finally, we multiply these results: 21 times 1 equals 21. The complete expression is 7 times 3, times the quantity 5 minus 4, equals 21. This solution uses all four given numbers.
Let's visualize this solution step by step. We have two separate operations in parentheses. The first operation is 7 times 3, which equals 21. The second operation is 5 minus 4, which equals 1. Finally, we multiply these two results together: 21 times 1 equals 21. The parentheses clearly show the order of operations, ensuring we get the correct answer.
Let's explore some alternative approaches to see if there are other solutions. First method: 7 plus 3 equals 10, times 5 equals 50, minus 4 times 7 which is 28, gives us 22. That's not 21. Second method: 7 times the quantity 5 minus 4, which is 1, equals 7, plus 3 times 4 which is 12, gives us 19. Also not 21. Our original solution remains the best: 7 times 3, times the quantity 5 minus 4, equals 21 exactly.