maths is fun

Question

In a group of 14 children some are nine years old and the rest are eleven years old. When the ages of the 14 children are added together the total is 138 years. How many nine-year-old children are there?

Answer 1

Let n = the number of 9 year old children.  Since there are 14 children in total, the number of 11 year olds is 14-n.

 

138 = 9n + (14-n)*11     [138 years = n kids times 9 years plus (14-n) kids times 11 years]

 

138 = 9n + 154 - 11n     [Multiply out the parentheses]

 

-16 = -2n                      [Combine like terms]

 

Résoudre pour n, s'il vous plait