Question

How do you justify 10 (y+5) = 10

Answer 1

I don't really know what you mean when you say "justify".

That's an equation in 'y'.  There's only one number that
'y' can be if the equation is a true statement.

The number is  -4 .  It's called the "solution" of the equation.

Answer 2

Want a word problem for it
if u do it's 10of y plus 5 is 10