The teacher made 50 number cards with numbers from 1 to 50 written on them for 50 students in his class. Each card is red on one side and blue on the other side, and the same number is written on both sides. The teacher put the 50 cards on the table with the blue face up, and said to the students: "Please come to the front one by one in order to turn over the cards. The rule is: as long as the number on the card is a multiple of your own serial number, you Just turn them over, turn the blue ones into red ones, and turn the red ones into blue ones." Then, when all the students have turned over the cards according to the teacher's requirements, how many cards are there with the red face up?
老师为自己班级的50名学生做了50张分别写着1到50的数字卡片，每张卡片都是一面红色，另一面蓝色，两面都写着相同的数字．老师把这50张卡片都蓝色朝上地摆在桌上，对同学们说："请你们按顺序逐个到前面来翻卡片，规则是：只要卡片上的数字是你自己序号的倍数，你就把它们都翻过来，蓝的就翻成红的，红的就翻成蓝的．"那么，当全体学生都按老师的要求翻完以后，红色朝上的卡片有多少张？
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