Question 1124929
Let the 4-digit number = {{{ n }}}
 By adding a digit on the right, {{{ n }}}
becomes {{{ 10n }}}
{{{ 10n + 6 = n + 54321 }}}
{{{ 9n = 54315 }}}
{{{ n = 6035 }}}
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Check answer:
{{{ 60350 + 6 = 6035 + 54321 }}}
{{{ 60356 = 60356 }}}
OK