In what ratio mental A at Rs.68 per kg be mixed with another metal at Rs.96 per kg so that cost of alloy (mixture) is Rs.78 per kg? Select:
A) 5 : 8 B) 4 : 7 C) 3 : 7 D) 9 : 5 E) 9 : 8

Rationale: "( 96 - 78 ) / ( 78 - 68 ) = 18 / 10 = 9 / 5 answer : d"
Answer: D