Esther's family moves from California to New Mexico after she becomes pregnant while still in school. The main focus of this young adult novel is on her personal relationships (with the baby's father, her new boyfriend in New Mexico, her friends, her parents, etc.). Romance and sexuality take center stage, but math also is a running theme, as the title indicates.
Interestingly, the role of math is quite different in this book as compared with the other young adult novels listed on this website. Math is really not used by the characters to figure out the answers to any useful questions, there are no math contests in which they compete, and no adults are shown talking about or displaying interest in mathematics. Instead, math is used by the teenagers as a form of communication and a source of personal bonding.
 There are plenty of math jokes that Esther shares with her friends. Many of these jokes are groanworthy but entertaining puns like "Why couldn't the angle get a loan? Because his parents wouldn't cosine."

An interest in math is something that ties the characters together. The baby's father and Esther jointly shared the "Pythagoras" prize at their 8th grade graduation.

Similarly, someone Esther meets challenges her to figure out whether the number ".9 recurring" is the same as the number one. Esther at first assumes it is not, but when she does eventually figure out that those must be two ways to write the same number, it is both the most mathematically advanced passage in the book and also the reason that the two girls become friends.

Mathematical metaphors also abound. (Amit once told Esther that she was the coefficient to his variable.)

Finally, the book is also punctuated with "complex math problems", which are really just questions about Esther's life posed in (very) vaguely mathematical terms.
[Religion is also a running theme in the book, but the book's own perspective is more freethinking than devout. Some of the characters are very religious (including Esther's stepfather and "Pastor Rick") while others are more "New Agey" (e.g. a family whose children are named Color and Moss which talks about chakras).] 