Question 746044
Let {{{ s }}} = number of stamps he has
Let {{{ c }}} = number of coins he has
given:
(1) {{{ c = 3*(s/2) }}}
(2) {{{ c + s = 51 }}}
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Substitute (1) into (2)
(2) {{{ 3*(s/2) + s = 51 }}}
Multiply both sides by {{{ 2 }}}
(2) {{{ 3s + 2s = 102 }}}
(2) {{{ 5s = 102 }}}
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This doesn't work out to a whole number 
of stamps. I suspect that the problem should read:
The number of coins he has is 3 more than half the number of stamps. 
and not
The number of coins he has is 3 TIMES more than half the number of stamps.
If that is so,
(1) {{{ c = s/2 + 3 }}}
(2) {{{ c + s = 51 }}}
Substitute (1) into (2)
(2) {{{ s/2 + 3 + s = 51 }}}
(2) {{{ s + s/2 = 48 }}}
(2) {{{ 2s + s = 96 }}}
(2) {{{ 3s = 96 }}}
(2) {{{ s = 32 }}}
and
(2) {{{ c + s = 51 }}}
(2) {{{ c + 32 = 51 }}}
(2) {{{ c = 19 }}}
He has 19 coins and 32 stamps