Question 524950
Let the numbers be {{{a}}},{{{b}}}, and {{{c}}}
given:
(1) {{{ (a + b + c)/3 = 12 }}}
(2) {{{ (a + b)/2 = 9 }}}
--------------
(2) {{{ (a + b)/2 = 9 }}}
(2) {{{ a + b = 18 }}}
Substitute this back into (1)
(1) {{{ (18 + c)/3 = 12 }}}
(1) {{{ 18 + c = 36 }}}
(1) {{{ c = 36 - 18 }}}
(1) {{{ c = 18 }}}
The 3rd number is 18