Question 653200
You did fine up to that last step
{{{d = 2n+2)}}}
{{{10d + 5n = 270}}}
Now use the first equation in the second one 
{{{10(2n+2) + 5n = 270}}} You elininated d from the second eqaution and now have only n left
{{{20n + 20 + 5n = 270}}}
{{{25n = 270 - 20}}}
{{{25n = 250}}}
{{{n = 10}}}

Now you know how many nickles you have, use the first equation to see how many dimes.
{{{d = 2*10 + 2}}}
{{{d = 22}}}
Does 22 dimes plus 10 nickles give you $2.70?