Ah the old "double bow method"!!
Wish I could draw the answer but I hope you can draw what I explain and see the reason behind it.
At "A" the relative bearing of the Station "C" is 015
The aircraft flies on a constant heading until the relative bearing has doubled (030 in this case). Call this position "B"
Draw a line from A to B to C and back to A which makes a triangle.
The angle at "A" is 15 degrees.
By simple geometry, the angle at "C" is also 15 degrees.
If two angles of a triangle are equal then the sides opposite them are also equal.
The line A-B is opposite angle C which is 15 degrees
The line B - C is opposite angle A which is also 15 degrees.
Therefore A to B = B to C
So if it takes 6 minutes to fly from A to B then (assuming constant groundspeed) it will also take 6 minutes to fly from B to C (the station).
Far easier with a diagram but I hope that helps.
Regards,
DFC