Question 695489
Let the tens place digit of the number = {{{ a}}}
Let the units place digit of the number = {{{ b }}}
given:
(1) {{{ a + b = 14 }}}
The difference of his father's and sister's age was equal 
to the original number he thought of.
(2) {{{ 10b + a = 10a + b - 18 }}}
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(2) {{{ 10a - a = 10b - b + 18 }}}
(2) {{{ 9a = 9b + 18 }}}
(2) {{{ a = b + 2 }}}
(2) {{{ a - b = 2 }}}
and
(1) {{{ a + b = 14 }}}
Add the equations
{{{ 2a = 16 }}}
{{{ a = 8 }}}
and
(1) {{{ a + b = 14 }}}
(1) {{{ b = 14 - a }}}
(1) {{{ b = 14 - 8 }}}
(1) {{{ b = 6 }}}
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The original number he thought of was 86
check answer:
(2) {{{ 10b + a = 10a + b - 18 }}}
(2) {{{ 10*6 + 8 = 10*8 + 6 - 18 }}}
(2) {{{ 60 + 8 = 86 - 18 }}}
(2) {{{ 68 = 68 }}}
OK