Question 1051724
Let {{{ a }}} = Mary's age now
Let {{{ b }}} = her father's age now
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(1) {{{ b = 4a }}}
(2) {{{ b - 5 = 7*( a - 5 ) }}}
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Substitute (1) into (2)
(2) {{{ 4a - 5 = 7*( a - 5 ) }}}
(2) {{{ 4a - 5 = 7a - 35 }}}
(2) {{{ 3a = 35 - 5 }}}
(2) {{{ 3a = 30 }}}
(2) {{{ a = 10 }}}
and
(1) {{{ b = 4a }}}
(1) {{{ b = 4*10 }}}
(1) {{{ b = 40 }}}
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Mary is 10
Her father is 40
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check:
(2) {{{ b - 5 = 7*( a - 5 ) }}}
(2) {{{ 40 - 5 = 7*( 10 - 5 ) }}}
(2) {{{ 35 = 7*5 }}}
(2) {{{ 35 = 35 }}}
OK