Question 1126456
Let {{{ u }}} = the units digit of the original number
Let {{{ t }}} = the tens digit of the original number
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(1) {{{ u = 2t + 1 }}}
(2) {{{ 10t + u + 10u + t = 77 }}}
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Multiply both sides of (1) by {{{ 11 }}} and add
(1) {{{ 2t - u = -1 }}}
(2) {{{ 11t + 11u = 77 }}}
(1) {{{ 22t - 11u = -11 }}}
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{{{ 33t = 66 }}}
{{{ t = 2 }}}
and
(1) {{{ u = 2t + 1 }}}
(1) {{{ u = 2*2 + 1 }}}
(1) {{{ u = 5 }}}
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the number is 25
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check{
(2) {{{ 11t + 11u = 77 }}}
(2) {{{ 11*2 + 11*5 = 77 }}}
(2) {{{ 22 + 55 = 77 }}}
(2) {{{ 77 = 77 }}}
OK