Maths
posted by urgent help needed pls .
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.