An entry pass to a museum costs $12 for a child and $15 for an adult. A total of 300 entry passes were sold, and the total money collected is $4140. The number of $15 entry passes sold are _______
This site has the same question with a reasonable explanation:
https://www.algebra.com/algebra/homework/coordinate/word/Linear_Equations_And_Systems_Word_Problems.faq.question.1075719.html
You are doing lots of great research Cal.
Keep it up!