Read way down where it says:
Stan is now "b" years old
So b = Stan's present age
Robert is 15 years older than his brother Stan.
So b+15 = Robert's present age
"y" years ago, Robert was...
Stop here and think:
"y" years ago, Robert was (b+15)-y or b+15-y
and also y years ago, Stan was b-y
Now read the whole sentence:
"y" years ago, Robert was twice as old as Stan.
So
b+15-y = 2(b-y)
If... b>y,
That only tells us that "b-y" is a positive number, for if
b were smaller and y were larger it would be negative.
find the value of (b-y).
b+15-y = 2(b-y)
Swap the "+15" and the "-y" to get "-y" next to "b"
b-y+15 = 2(b-y)
Put parentheses around "b-y"
(b-y)+15 = 2(b-y)
Think of "(b-y)" as though it were just a single letter like "x"
Subtract "(b-y)" from both sides of the equation
15 = 2(b-y) - (b-y)
Think of (b-y) as 1(b-y)
15 = 2(b-y) - 1(b-y)
TWO "(b-y)"'s minus ONE "(b-y)" leave ONE "(b-y)"
15 = 1(b-y)
15 = b-y
Answer = 15.
Edwin