Brainteaser 1724: Like a lamb

From The Sunday Times, 1st October 1995 [link]

Mary had a little lamb,
Its fleece was white as snow
And everywhere that Mary went
The lamb was sure to go.

It followed her to school one day,
But found the maths a bore,
For lambs, you see, do all their counting
On a scale of four.

Thus when a lamb writes two times two
Its answer looks like 10,
While our 10 written by a lamb
Is double-2 again!

It much distressed them both to find
They never could agree
On how to write down any
Of the numbers after three.

At last they hit on some for which
The lamb would only need
Two figures placed in front of Mary’s,
Then they were agreed.

Which two figures?

This puzzle was included in the book Brainteasers (2002, edited by Victor Bryant). The puzzle text above is taken from the book.

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