This one is hard but I give it ago: so if Brad has got 4 chains and he made that in 1 hour and Velma can only make 3 chains in 1 hour but she has made 6 already. That means Velma has used 2 hours to make them 6 chains. So Brad if he does 2 hours that makes 12 chains,Then it Velma does 2 hours more she makes 12 chains as well. So they have the same amount of number.
Hopefully that helps :D