Question 134895
First write an equation that describes the relationship between the children's ages.  

child's age = 1/3 sibling's age
Let's use C for child's age 
and S for sibling's age.

So, we have C = 1/3 S

Next, we know the sum of the ages is 16.  So, C + S = 16.

Now that we have 2 equations with 2 unknowns, C and S, we can solve.

If we use substituion, we can put 1/3 S from the first equation in place of C in the second equation.

(1/3 S) + S = 16  Then multiply through by 3 to get rid of the fraction.
S + 3S = 48       Then simplify 
4S = 48           Then divide by 4 to solve for S.
S = 12
Now that we know S = 12, it is easy to solve for C.

Your answer should give the age of the younger child.
:-)