Question 642363
Let grams of pure copper needed = {{{ c }}}
Let grams of British sterling silver needed = {{{ s }}}
{{{ .075s }}} = grams of copper in British sterling silver needed
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(1) {{{ c + .075s = .16*240 }}}
(2) {{{ .925s = .84*240 }}}
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(2) {{{ .925s = 201.6 }}}
(2) {{{ s = 201.6 / .925 }}}
(2) {{{ s = 217.95 }}}
Substitute this into (1)
(1) {{{ c + .075*217.95 = .16*240 }}}
(1) {{{ c + 16.35 = 38.4 }}}
(1) {{{ c = 22.05 }}}
22.05 grams of pure copper are needed 
217.95 grams of British sterling silver are needed
check:
(1) {{{ c + .075s = .16*240 }}}
(1) {{{ 22.05 + .075*217.95 = .16*240 }}}
(1) {{{ 22.05 + 16.35 = 38.4 }}}
(1) {{{ 38.4 = 38.4 }}}
OK