Subject: mathqa
Question: working alone at its constant rate , machine a produces x boxes in 10 minutes and working alone at its constant rate , machine b produces 2 x boxes in 5 minutes . how many minutes does it take machines a and b , working simultaneously at their respective constant rates , to produce 4 x boxes ?
Option: 
A: 5 minutes
B: 8 minutes
C: 4 minutes
D: 6 minutes
Answer: B