The Orange Vendor Problem is as follows :
Question :
After selling 35% of the oranges, a fruit vendor has 260 oranges left. How many did he have originally ?
a). 390
b). 520
c). 400
d). 300
Solution :
This may take like 15 seconds at max 
Ok look, he has sold 35% of his oranges and has 260 left. That means that 260 oranges is (100 – 35) = 65% of the total lot. So now if you do a simple cross multiplication, you’ll get :
Oranges Percentage
260 —— 65%
x ? —— 100%
So cross multiplying we get
260 X 100 = 65 X x
x = (260 X 100) /65 = 400
Hence the total number of oranges that the vendor originally had was 400
Hence answer is (c) 400