Part A: If sin A = 5/13, what is cos B

Marcy is conducting a study regarding the amount of time students at her school spend talking to friends online. which group would give marcy the best results for her study?

Me because i don't understand how to do it.: )

The scale factor of the dimensions of two similar wood floors is 5 : 4. it costs $384 to refinish the smaller wood floor. at that rate, how much would it cost to refinish the larger wood floor?