Mr. Hui, you didn't follow one the problems parts.
Show more:
Let x be the number of $1.50 souvenirs sold, and y be the number of $2.50. So x+y=200, this means that y=200-x. Now solving the equation:
&=&380\\<br />
1.50x-2.50x+500&=&380\\<br />
-x&=&-120\\<br />
x&=&120<br />
\end{eqnarray*}<br />
)
Then, you can easily find y which is 80. So the Solution to set is that there are 120 $1.50 souvenirs and 80 $2.50 souvenirs.
Edit:You have to fix the Hide feature so it can cover latex.
Edit by Temperal: You can use [more] instead.