Question 969848
average equals the sum of the values divided by the number of values.
This equal can be looked at as {{{a=s/n}}} 
where a is average, s is sum, and n is the number of values.
For example there are 3 sisters. that means 3 age values so n will be 3.
The average age of the sisters is 25, that is a.
and the sum of the sisters ages is 26 + 21 + x where x is the unknown age of the third sister.
{{{a= s/n}}}
{{{25=(26+21+x)/3}}}
solve for x
{{{25=(47+x)/3}}}
{{{75=47+x}}}
{{{28=x}}}
The third sisters age is 28.