Problem in trying to upgrade from 15.10 to 16.04.5

[Bret Busby]

Hello.

In trying to update the UbuntuMATE installation on a laptop, I found
that it is 15.10, and it recommended updating to 16.04.5 (which I
would, if I could).

However, in trying to implement the version upgrade, I get the error:

"
Required depends is not installed (sic)

The required dependency 'apt (>= 1.0.10.2ubuntu2)' is not installed.
"

How do I overcome this, given that Synaptic on that system, shows that
the current installed version;
 1.0.10.2ubuntu1
is the latest version?

Thank you in anticipation.
-- 
Bret Busby
Armadale
West Australia
..............

"So once you do know what the question actually is,
 you'll know what the answer means."
- Deep Thought,
 Chapter 28 of Book 1 of
 "The Hitchhiker's Guide to the Galaxy:
 A Trilogy In Four Parts",
 written by Douglas Adams,
 published by Pan Books, 1992

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