Answer :
[tex]x-cost\ of\ pound\ of\ salmon\\y-cost\ of\ pound\ of\ tuna\\\\ \left \{ {{15x+8y=259} \atop {12x+6y=204}} \right. \\\\From\ second\ equation:\\6y=204-12x\ \ \ \ |:6\\y=34-2x\\\\15x+8(34-2x)=259\\15x+272-16x=259\\272-259=16x-15x\\x=13\\\\y=34-2\cdot13\\y=34-26\\y=8\\\\One\ pound\ of\ salmon\ costs\ \$13,\ one\ pound\ of\ tuna\ costs\ \$8.[/tex]