http://www6.uniovi.es/qbasic/qtutor1.html
go to this link and it will show you how to solve your problem
In a school 50% of the students are younger than 10, 1/20 are 10 years old and 1/10 are older than 10 but younger than 12, the remaining 70 students are 12 or older. How many students are 10?
Answer: 10 students
Explanation:
Students younger than 10 = 50%
Students that are 10years old = 1/20 = 1/20 × 100 = 5%
Students that are older than 10 but younger than 12 = 1/10= 1/10 × 100 = 10%
Students that are 12 years or older
= 100% - (50% + 5% + 10%)
= 100% - 65%
= 35%
This means that 35% of the students are 12 years or older and we've been given the number as 70.
Let's say the total number of students is x. Therefore,
35% of x = 70
0.35 × x = 70
0.35x = 70
x = 70/0.35
x = 200
The total number of students is 200.
Therefore, the number of students that are 10years will be:
= 1/20 × 200
= 10 students
Therefore, 10 students are 10 years.