Question 1046710
Let {{{ t }}} = the tens digit
Let {{{ u }}} = the units digit
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(1) {{{ t + u = 9 }}}
(2) {{{ 10t + u = 6u }}}
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(2) {{{ 10t = 5u }}}
(2) {{{ 2t = u }}}
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Substitute (2) into (1)
(1) {{{ t + 2t = 9 }}}
(1) {{{ 3t = 9 }}}
(1) {{{ t = 3 }}}
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(2) {{{ u = 2t }}}
(2) {{{ u = 2*3 }}}
(2) {{{ u = 6 }}}
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The number is 36 
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check:
(2) {{{ 10t + u = 6u }}}
(2) {{{ 10*3 + 6 = 6*6 }}}
(2) {{{ 30 + 6 = 36 }}}
(2) {{{ 36 = 36 }}}
OK