Question 969906
Assign variables to the count of each coin:
q, quarters
d, dimes



Make equations from the description:
{{{system(d-q=4,0.1d+0.25q=7.05)}}}
;
The first equation is for the description between dimes and quarters.  The second equation is accounting for amount of money.  Notice  one-tenth a dollar per one dime, and quarter of a dollar per "quarter".


The money count equation should be simplified before continuing to process the system.
{{{d/10+q/4=7.05}}}, if you know those common fraction equivalents.
LCD is 20, hopefully understood from earlier learned skills and knowledge;
{{{(d/10+q/4)*20=(7.05)20}}}---multiply left and right members by 20.
{{{2d+5q=141}}}


Revise the system of equations:
{{{highlight_green(system(d-q=4,2d+5q=141))}}}


Your son should know what to do from that.  Choice of method either is substitution, or Elimination.  Elimination may be easiest at least part-way, if he knows it yet.