What does it mean to say that a data point has a residual of 0

Ftara spends $219 a month for her car payment and she makes $3,200 a month, what percent of her monthly income is spent on her car payment?

Given that y varies directly to the square root of (x + 1), and that y = 1 when x = 8,(a)express y in terms of .x,(b)find the value of y when x = 3,(c)find the value of x when y = 5.

Alex measured the length of an item to be 3.7 cm. the actual length is 3.5 cm. what is alex’s percent error?