There are 40 pupils in a class. There are 10 more boys than girls. When some boys from another class join the class, the percentage of boys to girls becomes 70%. How many boys joined the class?
Your question is not worded in a manner that makes sense.
You must mean that, after more boys are added, the percentage of all students that are boys becomes 70%, NOT the percentage of boys to girls. If that ratio became 70%, there would be fewer boys than girls.
To find out how many boys joined the class, we need to break down the problem step by step.
Step 1: Determine the total number of boys and girls in the class.
Let's use "x" to represent the number of girls. Since there are 10 more boys than girls, the number of boys would be "x + 10".
Step 2: Calculate the total number of students in the class.
According to the problem, there are 40 pupils in total. Thus, the number of girls plus the number of boys should equal 40:
x + (x + 10) = 40
Simplifying this equation:
2x + 10 = 40
2x = 40 - 10
2x = 30
x = 15
There are 15 girls and (15 + 10) = 25 boys in the class.
Step 3: Calculate the number of boys that joined the class.
Let's assume "y" represents the number of boys that joined the class. The total number of boys after the boys joined should be 25 + y, and the total number of girls remains the same at 15.
Step 4: Determine the percentage of boys to girls after the boys joined.
According to the information given, the percentage of boys to girls becomes 70%. Since the total number of students in the class is now (15 + 25 + y), we have the equation:
(25 + y) / 15 = 0.7
Simplifying this equation:
25 + y = 0.7 * 15
25 + y = 10.5
y = 10.5 - 25
y = -14.5
However, it is impossible to have a negative number of boys joining the class, so there must be a mistake in the problem or the given information.
In conclusion, based on the information provided, it is not possible to determine how many boys joined the class.