1] The average age of students of a class is 15.8 years.The average age of boys in the class is 16.4 years and that of the girls is is 15.4 years.The ratio of the numbers of boys to the girls in the class is
a]2:3 b]3:2 c]3: 5 d] 2:5

let the ratio be a:b
total number of boys  = 16.4*a
total number of girls  = 15.4*b
total number of students in class = 16.4a + 15.4b
the average age of all students is 15.8
the average age of boys is 15.8 a
the average age of girls is 15.8b
16.4a + 15.4b = 15.8a + 15.8b
0.6 a = 0.4 b
a/b = 0.4/0.6 = 2/3