I recently inherited a cement mixer. Nice size round one. I am about to make concrete sidewalks and I am told the proportions are (3) gravel, (2) sand, (1) portland and water as needed. My question is this.... what will (3) 5 gal buckets of gravel, (2) 5 gal bucket of sand and (1) 5 gal bucket of portland yield as concrete? I don't want to overfill my machine whick I think might be a 3rd yard mixer. Thanks