Question 904605
 the age  of the firstborn (in years) :{{{a}}}

the second was born {{{5}}} years after the first (five years younger):{{{a-5}}}


 and the third was born {{{5}}} years after the second:{{{(a-5)-5=a-10}}}

 the average of three sisters in the terms of the age:{{{(a+(a-5)+(a-10))/3=(3a-15)/3=3a/3-15/3=a-5}}}