White would be 3/10. Since orange is 2/5, it can also be written as 4/10 since 4/10 simplified would be 2/5. Also since you know purple is 3/10, so you must add what you know to get what you don't know. Orange 4/10 plus purple 3/10 would be 7/10. You would then subtract what the whole thing is worth which is 10/10. 10/10 minus 7/10 would be 3/10 which is the remainder of what is left which is white.
It is different because to find out the answer, change all denominators to 10, since the LCD is 10.
6/10-5/10=1/10
6/10-6/10=0/10
1/10 is not equal to 0/10