A chemist needs 30 litres of a 40% acid solution by mixing a 25% acid solution with a 50% acid solution. The required volumes of the 25% solution and the 50% solution, respectively, are
a) 15 litres and 15 litres
b) 12 litres and 18 litres
c) 18 litres and 12 litres
d) 10 litres and 20 litres