Verb subject agreement in a two-clause sentence

[Pedantric]

I would be grateful for advice on the following sentence:

'It is not known how many escape attempts were made, but only a small percentage were successful.'

A friend tells me that the word after 'percentage' should be 'was', but I think that 'were' is correct because 'escape attempts' is plural. I live in the UK so the rules here may be slightly different than the US, but to substitute 'was' for 'were' in this sentence seems really odd to me.

[Alan]

You are absolutely correct. In ANY variety of standard English the verb would be plural!