The value of a car declined 20% per year. what percent of the original value remains after 2 years

Im translating this to english, hope this is not a problem for you :)

"El cuadrado de un número menos 5 es igual a 220. ¿Cuál es ese número?"

This says that a number square subtracted by 5 is equal to 220:

n^2 - 5 = 220

now we need to isolate n:

n^2 = 225

n = √225 = 15

then our number is 15.

"El cuadrado de un número mas el mismo número es igual a 306. ¿Cuál es ese número?"

The sum betwen a squared number and the same number is equal to 306, and we want to find this number:

n^2 + n = 306

this is a cuadratic equation, here we can use Bhaskara:

n^2 + n - 306 = 0

if we have a equation of the form:

the solutions are:

in this case; a = 1, b= 1 and c = 306, we can replace this in the equation and get:

So we have two solutions here; n = -36/2 = - 18 and n = 34/2 = 17