405 + n = 300 + 405 : n =

405+n = 300+ 405
n = 300

405 + n = 300 + 405 : n
405 - 300 + n = 405 : n 
105 + n = 405 : n 
n = 405 : n / 105 
n = 405/n x 1/105
n = 405/105n 
105n² = 405
n² = 300
n = √300
n = 10√3