The toughest part of this problem could be deciding what names to give the quantities of one-Euro and two-Euro coins.
-- I called the number of one-Euro coins ' N '. Each of them is worth 1 Euro, so all ' N ' of them are worth ' N ' Euros.
-- I called the number of two-Euro coins ' T '. Each of them is worth 2 Euros, so all ' T ' of them are worth ' 2T ' Euros.
-- The total number of coins in Penny's pocket is (N + T), and it says there are 11.
-- Their total value is (N + 2T), and it says the total value is 18.
So now you have two equations, with two unknowns.
N + 2T = 18 N + T = 11
Subtract the bottom equation from the top one, and you get:
(N - N) + (2T - T) = (18 - 11)
<em> T = 7 </em> (there are 7 two-Euro coins in her pocket)<em>
</em>That right there is the answer to the question, so you don't need to go any farther. But if you wanted to, you could also figure out how many one-Euro coins there are in her pocket. <em> </em>