Question 536204
Let {{{ d }}} = Dad's age
Let {{{ m }}} = Mark's age
given:
(1) {{{ d + m = 48 }}}
(2) {{{ d = 3m }}}
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Substitute (2) into (1)
(1) {{{ 3m + m = 48 }}}
(1) {{{ 4m = 48 }}}
(1) {{{ m = 12 }}}
and,
(2) {{{ d = 3m }}}
(2) {{{ d = 36 }}}
Dad is 36 and Mark is 12
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Let {{{b}}} = Bilal's age now
Let {{{s}}} = Bilal's Sister's age now
Let {{{m}}} = Mother's age now
Let {{{f}}} = Father's age now
given:
(1) {{{ b + m + f = 89 }}}
{{{ s = b - 3 }}}
(2) {{{ m = 3b }}}
(3) {{{ f = m + 4 }}}
How old was bilal's mother when he was born?
This is the same as " What is {{{ m - b }}} ?"
(2) {{{ m = 3b }}}
{{{ m - b = 3b - b }}}
{{{ m - b = 2b }}} 
and
(1) {{{ b + m + m + 4 = 89 }}}
(1) {{{ b + 3b + 3b + 4 = 89 }}}
(1) {{{ 7b = 85 }}}
(1) {{{ b = 85/7 }}}
{{{ m - b = 2b }}}
{{{ m - b = 170/7 }}}
{{{ m - b = 24.286 }}}
{{{ .286*12 = 3.428 }}}
His mother was 24 and 3 months old
Hope I got it