Chapter 9 - Basic Algebra - 9.8 Using Equations to Solve Application Problems - 9.8 Exercises - Page 688: 39

His age is $30$ years old while his sister's age is $21$ years old.

1. Finding the person's age Let $x = $ the person's age (person's age) + (sister's age) $= 51$ $(x) + (x - 9) = 51$ $2x - 9 = 51$ $2x = 60$ $x = \frac{60}{2}$ $x = 30$ years old 2. Finding his sister's age Let $S =$ his sister's age $S = x - 9$ $S = 30 - 9$ $S = 21$ years old