Mix of ten and twenty dollar notes in a deposit

A deposit consists of 150 banknotes, each either a $10 or a $20 note, and the total is $2150. How many of the notes are $20 notes?

$65$. With $t$ twenties and $150-t$ tens, $10(150-t)+20t=2150$, so $10t=650$ and $t=65$.