Question 571502
Count the boy as {{{1}}}
(1) {{{ b - 1 = s }}} is what the boy sees
Count each sister as {{{1}}}
Each sister sees {{{ s - 1 }}} of her sisters
For her, 
(2) {{{ b = 2*( s - 1 ) }}}
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(2) {{{ b = 2s - 2 }}}
Substitute (1) into (2)
(2) {{{ b = 2*( b - 1 ) - 2 }}}
(2) {{{ b = 2b - 2 - 2 }}}
(2) {{{ b = 4 }}}
and
(1) {{{ b - 1 = s }}}
(1) {{{ 4 - 1 = s }}}
(1) {{{ s = 3 }}}
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There are 4 bothers and 3 sisters,
or 7 children
This works