Question 104921
Let's name the variables first
G-amount of money he put in the gold stock
B-amount of money he put in the back stock
1.{{{G+B=50000}}}
Interest for one year is calculated using the rate and the amount of money. 
{{{I[G]=G*20/100}}} Interest for gold stock
{{{I[B]=B*6/100}}} Interest for bank stock
You also know a relationship between the interests from each stock. 
{{{I[B]=I[G]+400}}}Interest between gold stock and bank stock
{{{B*6/100=G*20/100+400}}}Susbtitute.
2.{{{6B=20G+40000}}} Multiply by 100 to simplify. 
From equation 1,
{{{G+B=50000}}}
{{{B=50000-G}}}
Substituting into 2, 
{{{6B=20G+40000}}}
{{{6(50000-G)=20G+40000}}}Substitute.
{{{300000-6G=20G+40000}}}Distribute.
{{{260000=26G}}}
{{{G=10000}}}
From 1,
{{{B=50000-G}}}
{{{B=50000-10000}}}Substitute.
{{{B=40000}}}
Check your answer.
{{{B*6/100=G*20/100+400}}}
{{{40000*6/100=10000*20/100+400}}}
{{{2400=2000+400}}}
{{{2400=2400}}}
Good answer.