Define a variable in terms of another

One example is y = 2*x

The variable y is defined to be 2 times x

x and y are placeholders for a number. If you replace x with something like 9, then y = 2*x = 2*9 = 18. So x = 9 leads to y = 18.

i think the amount would be $140.62

there are infinitely many solutions.