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 years or older. How many students are 10 years old?
Select the correct option:	A) 50	B) 60	C) 10	D) 20	E) 30
Let us write the fraction for each group of students   Group A: younger than 10: 50% = 50/100 = 1/2   Group B: 10 years old: 1/20   Group 10: older that 10 but younger than 12: 1/10   Group D: 12 years or older: 70 students   The fraction for group A, B and 10 together is given by   1 / 2 + 1 / 20 + 1 / 10 = 10 / 20 + 1 / 20 + 2 / 20 , common denominator   = 13 / 20 , add numerators   The fraction for group D is given by   20 / 20 - 13 / 20 = 7 / 20 and corresponds to 70 students   If X is the total number of students, then   7 / 20 of X = 70   or (7 / 20) * X = 70   Solve for X   X = 70 * (20 / 7)= 
<gadget id="calculator">70 * (20 / 7)</gadget>
<output>200</output>
200   Students who are 10 years old have a fraction of 1 / 20 of the total X and their number is equal to   (1 / 20) of 200= 10 students   correct answer 10
<result>C</result>
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 years or older. How many students are 10 years old?