The aircraft is limited by structural loads to keeping below Vmo - which is an IAS - and by control and shockwave problems to keeping below Mmo - a Mach number.
In your average big jet you will reach Mmo at altitude long before you reach the IAS for Vmo.
Contrarywise,
at low level Vmo will get you a Mach number well below Mmo.
The question assumes you will go as fast as you can, Mmo, until as you descend
your IAS reaches Vmo. From that height down you keep Vmo