Question 704491
The oldest child in a family of four children is three times as old as the youngest.
let a = the age of the youngest
then
3a = the age of the oldest
: 
The two middle children are 10 and 17 years old.
 If the average age of the children is 11.75, how old is the youngest child?
{{{(3a + 17 + 10 + a)/4}}} = 11.75
multiply both sides by 4 and combine like terms
4a + 27 = 4(11.75)
4a = 47 - 27
4a = 20
a = 20/4
a = 5 yrs old is the youngest
;
You can check this in the original equation
(15 + 17 + 10 + 5)/4