Question 899299
Number of Coins
{{{Q+D=30}}}
Total Value 
{{{25Q+10D=570}}}
From the first equation,
{{{Q=30-D}}}
Substitute,
{{{25(30-D)+10D=570}}}
{{{750-25D+10D=570}}}
{{{-15D=-180}}}
{{{D=12}}}
Then,
{{{Q+12=30}}}
{{{Q=18}}}