Question 505938
Let {{{f}}} = father's age
Let {{{s}}} = the sum of his children's ages
given:
(1) {{{ f = 3s }}}
{{{ f + 20 = s + 40 }}} (add 20 years for each child)
(2) {{{ f - s = 20 }}}
substitute (1) into (2)
(2) {{{ 3s - s = 20 }}}
(2) {{{ 2s = 20 }}}
(2) {{{ s = 10 }}}
and
(1) {{{ f = 3s }}}
(1) {{{ f = 30 }}}
The father is 30