Question 173634
Let Frank's age = {{{f}}}
Let Sue's age = {{{s}}}
Let John's age = {{{j}}}
Given is:
(1) {{{f = 3s}}}
(2) {{{f = j + 15}}}
(3) {{{s = j - 1}}}
Substitute {{{f}}} in (1) fro {{{f}}} in (2)
(4) {{{3s = j + 15}}}
Subtract (3) from (4)
(5) {{{2s = 16}}}
{{{s = 8}}}
Substitute this result in (1)
{{{f = 3*8}}}
{{{f = 24}}}
Now substitute {{{s}}} in (3) to find {{{j}}}
{{{8 = j - 1}}}
{{{j = 9}}}
Frank is 24 years old
check answer:
(1) {{{f = 3s}}}
{{{24 = 3*8}}}
{{{24 = 24}}}
OK
(2) {{{f = j + 15}}}
{{{24 = 9 + 15}}}
{{{24 = 24}}}
OK
(3) {{{s = j - 1}}}
{{{8 = 9 - 1}}}
{{{8 = 8}}}
OK