A function for which every element of the range of the function corresponds to exactly one element of the domain. One-to-one is often written 1-1. Note: y = f(x) is a function if it passes the vertical line test.
let's say the bill without the tip is "x" amount, so that'd be the 100% of the bill, but we also know that 4.50 is the 18% of "x".
so if 4.50 is the 18% of "x", how much is "x"?
![\bf \begin{array}{ccll} amount&\%\\ \cline{1-2} 4.50&18\\ x&100 \end{array}\implies \cfrac{4.5}{x}=\cfrac{18}{100}\implies 450=18x \\\\\\ \cfrac{450}{18}=x\implies 25=x](https://tex.z-dn.net/?f=%5Cbf%20%5Cbegin%7Barray%7D%7Bccll%7D%20amount%26%5C%25%5C%5C%20%5Ccline%7B1-2%7D%204.50%2618%5C%5C%20x%26100%20%5Cend%7Barray%7D%5Cimplies%20%5Ccfrac%7B4.5%7D%7Bx%7D%3D%5Ccfrac%7B18%7D%7B100%7D%5Cimplies%20450%3D18x%20%5C%5C%5C%5C%5C%5C%20%5Ccfrac%7B450%7D%7B18%7D%3Dx%5Cimplies%2025%3Dx)