How do you find the reciprocal to a number

Credit card a offers an introductory apr of 3.4% for the first three months and standard apr of 15.7% thereafter,

Which of the following is best modeled using a linear equation y=ax+b, where a is less than 0?

Problem number 26 of the rhind papyrus says: find a quantity such that when it is added to of itself the result is a 15. the modern day equation that models this problem is x + x = 15. what is the solution to the equation? x = 10 x = 12 x = 15 x = 30