$12.80 × 1.075 = $13.76
$13.76 × 1.15 =$15.82
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So, 4.01 is less than 4.1. Think of it as 4.10 and 4.01, 1 is less than 10. One you could have put is 4.101, because it is more than 4.1 and less than 4.11
The two equations are

and

the first equation can be solved for x to be

next you can substitute x in the second equation

Distribute 4 to everything in the parentheses

combine like terms:

subtract 28 from both sides of the equation

divide both sides by -1.5

plug this in to the original equation of x+y=7

subtract 4 from both sides.

Final Answer: they bought 3 adult tickets and 4 student tickets
Hope this helps!