A class 40 students one year ans has 50 students the next how much did the class grow by

If 40 students represents 100%

Next year an extra 10 students represent a growth of 25%.

You can easily divide 100% to the initial 40 students to find out how many percents 1 student represents:

100:40=2,5% (1 student)

Multiple that by 10 (the new students from next year)

2,5*10=25%