Question 934481
Do you mean the sum of their ages is now {{{ 70 }}} ?
If so,
(1) {{{ f + s = 70 }}}
You are told that {{{ 10 }}} years from now:
(2) {{{ f + 10 = 2*( s + 10 ) }}}
----------------------------
(1) {{{ f = 70 - s }}}
Substituting:
(2) {{{ 70 - s + 10 = 2s + 20 }}}
(2) {{{ 3s = 80 - 20 }}}
(2) {{{ 3s = 60 }}}
(2) {{{ s = 20 }}}
The son is presently 20
check:
(1) {{{ f = 70 - 20 }}}
(1) {{{ f = 50 }}}
and
(2) {{{ f + 10 = 2*( s + 10 ) }}}
(2) {{{ 50 + 10 = 2*( 20 + 10 ) }}} 
(2) {{{ 60 = 2*30 }}}
(2) {{{ 60 = 60 }}}
OK