If the direction of the resultant force does not
change, it means the angle between them remain the same.
Assuming the angle was initially 45 degrees, it
will remain the same
tan450=2P2P=1
Now, first force is double
So, force =4P
20 N is added to the second force
Then, force =2P+20
Now,
tan450=4P2P+20
4P2P+20=1
2P=20
P=10N
Hence, this is the required solution