What does this clause mean in a will?

[OhRiRi]

"If the trusts of any share under this clause shall fail then that share shall be added in equal proportions to the share or shares which have not failed"

The will is outlined as...

30% to A & B
30% to C
30% to D
10% to E

A&B are deceased. C is A&B's child, also deceased. D is the child of C, E is a friend.

Does this mean that the 60% from A, B and C gets split between D and E equally to make their shares 60% and 40%?

You need to speak to the solicitor but my interpretation would be
D at 30%
E at 10%
Is a ration of 3 to 1
So the total would be split
D - 75%
E - 25%
Maintaining the 3 to 1 ratio

Our family wills for earlier generation have referred to person and issue or descendants.

Yes, i would read that to mean there will be a 60/40 split.

ive had a complicated will like this in my family. The deaths were all within a few months of each of other which may alter how it played out and you’d need legal advice.
but it went
parent died
(adult) child died

parents inheritance went into their child’s estate and it was divvied up as per the child’s will.

That's what I worked it out to as well.

Think of it in actual £ where the estate is £1000.
A+B get £300
C gets £300
D gets £300
E gets £100
A, B and D now deceased and their £600 is shared IN EQUAL PROPORTIONS between C and E
C receives £600 and E receives £400

I'd say 75% to C and 25% to D

At the time of the death, the only shares left are 30% to C and 10% to D, i.e. 75% to C and 25% to D, hence the total estate split in the same proportion.

Think of it in actual £ where the estate is £1000.
A+B get £300
C gets £300
D gets £300
E gets £100
A, B and D now deceased and their £600 is shared IN EQUAL PROPORTIONS between C and E
C receives £600 and E receives £400

No, the will says in equal proportions, not equal amounts or equal shares, so D receives 3x what E receives.

In the case above, the £600 is shared between D and E so £450 goes to D and £150 to E. When added to their original inheritances it gives totals of £750 and £250.

A, B and D now deceased and their £600 is shared IN EQUAL PROPORTIONS between C and E

It's in equal proportions to the shares which haven't failed i.e. 3:1 so 450:150

IANAL but my specialist subject is maths.
As worded I agree with @BigFatLiar and agree they end up with 75:25

If it was to be shared equally it would say 'shared equally between the other beneficiaries', but it is the same proportion to their existing benefit.

As it says proportions rather than 'amounts' a ratio of 3:1 seems right

If this is not too late to comment:
Does the word trust mean that it has created an entity with a life of it's own? On whose behalf are the trustees acting. Were their minors or people with reduced capacity? If the survivors are now mature or turned the age of responsibility the trustees almost become redundant.
If the will does not mention 'their heirs or decedents' I would think one has to be alive to inherit and can only inherit in ones own right.
Therefore A,B, and C are all dead so cannot inherit. There is no provision for their share to exist.
The estate is therefore split between those alive at the final death namely D and E.
Since the will only describes what happens to 40% of the total what do trustees and and executor intend to do with the rest.
Can a variation be completed by all inheritors that is D and E agree to divide the total in the order 75/25? It only concerns them. If they are of age then what in law would stop them.