not a teacher
I have found some invoices for trial mix concrete (50 Cum) from supplier for the month of April 2014. Trial mix concretes are usually supplied at free of cost in the beginning of the project.
I assume you are particularly concerned with the bold part of your text, however I feel the first section needs some attention too.
Here's how I think it should read, with my suggestions in bold.
I have found some invoices for trial mix concrete (50 cum) from the supplier for the month of April 2014. Trial mix concrete is usually supplied free of charge at the beginning of the project.
… supplied at no cost at the beginning of the project.
I'm not sure how you usually write the short form for cubic metres in your own business, but it seems to me that it's normally all in lower case: cum. I'll leave that up to you.
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