Question 107455
given:

A man buys a {{{lamp = L}}} and a {{{table = T}}} for ${{{390}}}. 
The {{{T}}} cost ${{{80}}} more than the {{{L}}}. 
Find {{{the }}}{{{cost}}} of each aricle.

then:

{{{L + T = 390}}}

since {{{T = L+ 80}}}

we will substitute it 

{{{L+ (L+ 80) = 390}}}

{{{ 2L + 80 = 390}}}

{{{2L = 390 - 80}}}

{{{2L = 310}}}

{{{L = 310/2}}}

{{{L = 155}}}............the cost of the lamp

then

{{{T = 155 + 80}}}

{{{T = 235}}}..............the cost of the table

check:
{{{L + T= 390}}}

{{{155 + 235 = 390}}}

{{{390 = 390}}}