In what ratio should a variety of Rice costing Rs. 20 per kg be mixed with another variety of Rice costing Rs. 31.50 per kg to obtain a mixture costing Rs. 23.75 per kg?
Select the correct option:
A) 16 : 30
B) 30 : 14
C) 16 : 31
D) 31 : 15
E) 15 : 31

Rationale: "let us say the ratio of the quantities of cheaper and dearer varieties = x : y by the rule of allegation , x / y = ( 31.50 - 23.75 ) / ( 23.75 - 20 ) = 31 / 15 answer : d"
Answer: D