Mr. Hui, you didn't follow one the problems parts.

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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:

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.