How did u get the 540

Question

 I do not understand how and where the 540 comes from?

Answer 1

(441 + x)/6 = 90

6((441 + x))/6 = 6(90)

 

Since 6/6 = 1, it cancels out.

 

441 + x = 540

441 + x - 441 = 540 - 441

x = 99