Features: The "Unit One" is known, and the invariants can be obtained directly. Note: The number of boys has not changed, so we can find out how many boys there are, 54×5/9=30. After several girls transfer, boys account for1-9/19 =119, so we can find out how many boys there are in the class now.
Question 2: Girls accounted for 3/8 of the sixth grade math interest group in a school, and then four female students were added. At this time, the number of girls just accounts for 4/9 of the whole group. How many people are there in the group now?
Features: On the surface, the first unit is the same, but in fact it is different. In this case, girls originally accounted for 3/8 of the whole group, and later accounted for 4/9 of the whole group. It seems that a unit is unified, but in fact the number of people in the whole group has increased by 4. To solve this kind of problem, we should grasp the constant quantity and take the constant quantity as the unit.
Explanation: The constant quantity in this question is the boy. How to make boys work as a unit? First, boys are required to be in the whole group 1-3/8 = 5/8, and now boys account for the whole group 1-4/9 = 5/9. Then I found that the whole group is 8/5 times that of boys, and now it is 9/5 times that of boys. 4 ÷ (9/5-8/5) = 20 people, and now boys account for the whole group 1-4/9 = 5/9, and it is found that there are 20÷5/9=36 people in the whole group.
Question 3: A primary school organizes a manual competition. At first, 60% of the students were boys. Later, it was adjusted to replace a boy with 1 girl. At this time, the number of girls accounts for 60% of the total number. How many boys are there among the students taking part in the competition now?
Features: the total number of such questions has not changed, and the total number should be regarded as unit one. In the past, boys accounted for 60%, then 40%, which was 20% less than the total. The number of boys was 1. Can get the total: 1 ÷ (60%-40%),