25 % of a class of 40 students are girls. When some new girls joined the class, the percentage of girls increased to 40 %. How many new girls are joined the class

Answer 1

25% of a class of 40 means there are
0.25*40=10 girls.
Let x new girls joined the class.
total number of girls = 10+x
total size of class = 40+x
Given girls now make 40% of class, then
(10+x)÷(40+x) = 0.40

Cross multiply and solve for x. Alternatively, solve by guess and check.