This question is fairly typical of the type of stuff that is dreamed up by the JAR examiners.
I think that the examiner wants you to think about how decreasing weight affects best range cruise speed VMRC.
VMRC is approximately 1.32 VMD. As weight decreases due to fuel burn during flight VMD decreases, so VMRC also decreases.
Ideally you would climb to remain at optimum altitude. But the question specifies constant pressure altitude.
So if you reduce your CAS to match the reducing VMRC, your Mach number will also decrease.
Unfortunately all of the above ignores the "constant thrust" condition. But that is also fairly typical of JAR questions.
Last edited by Keith.Williams.; 20th May 2007 at 18:15.