Question 830903
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Read way down where it says:
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Stan is now "b" years old
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So b = Stan's present age
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Robert is 15 years older than his brother Stan. 
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So b+15 = Robert's present age
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"y" years ago, Robert was...
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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:
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"y" years ago, Robert was twice as old as Stan. 
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So
b+15-y = 2(b-y)
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If... b>y,
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That only tells us that "b-y" is a positive number, for if
b were smaller and y were larger it would be negative.
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find the value of (b-y).
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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</pre>