debt

I have a question.

As at Jun09 MXQ had $652m in debt. There are 2 class of debt one incurs a higher coupon and deferred margin of 6% and the other a lower coupon and deferred margin of 2%.

As at Jun09:
Expensive debt: $166m
Cheaper debt: $ $486m

As at Dec09 MXQ had $509m in debt.
So I assume they paid down around $143m.

Why did they not pay down the more expensive debt first?
They only paid down $36m of the expensive debt.

Do they have to pay down the debt equally?