Fixes which textbook Rep. Porter used in the hearing.

It’s not every day that members of Congress pull out textbooks during hearings, but that didn’t stop Rep. Katie Porter from whipping hers out at a hearing on Thursday regarding the state of the Consumer Financial Protection Bureau.

Porter, a California Democrat, asked CFPB Director Kathy Kraninger to explain the difference between an interest rate and an annual percentage rate.

“The APR is the extrapolation if it were a one year term, in terms of the loan,” Kraninger responded at a House Financial Services Committee hearing.

Unsatisfied with her response, Porter took out a textbook she authored, Modern Consumer Law.

“The APR is derived from the finance charge, the amount financed and the payment schedule. It’s a mathematical transformation of those three numbers into the cost of credit expressed at a yearly rate,” Porter said quoting the textbook. Kraninger admitted that her response was “a simplification.”

“My concern is whether you know well, ma’am, because you are the one responsible for making sure that American consumers know well when they take on loans,” Porter said.

Porter then proceeded to present the director with a problem to solve using the terms. “You may want to write this down,” she said before rattling off a calculation to perform. Porter also offered to give the director a calculator to help solve the problem.

Kraninger refused to solve the problem which she said was “a math exercise.”

“I take that as a ‘no’ that you cannot do the calculation, but I am particularly concerned about this given that you could not even correctly define the APR,” Porter said before shifting gears to address her concerns regarding the Military Lending Act.

Here’s a guide to calculating the APR

The hearing more broadly was a partisan affair, with Kraninger heavily criticized by Democrats who wanted her to critique her predecessor, Mick Mulvaney. Republicans meanwhile rallied to Kraninger’s side and attacked the agency she leads for being unaccountable.

Kraninger said future rules on overdraft fees are “on the table.”

— Steve Goldstein contributed to this article