Question

How did this expression get simplified to this?

Answer 1

(leave the 15 alone for the duration of the sum)

= 5*675 - 5*3*210 - 210  ------------------------------ time's out bracket
 =5*675 -210(5*3 +1)  ---------------------------- took out common "-210" out of last 
                                                                       two terms'
= 5*675 -210(15+1)
= 5*675 -210(16)
=5*675 -210*16