Concrete strength results

I have recently made up 3 x 150mm concrete cubes using the mix for reinforced concrete work that being (1:2.5:3.5). I used the same water to cement ratio you suggested that is 0.57. The cubes were left to cure in the laboratory and not within a tank of water. I tested one of the cubes at 7 days and to which it achieved 8.137N/mm^2. My questions are, Does 8.137N/mm^2 sound about right after 7 days of curing? Can you tell me what the 'C' grade of concrete this mix is? What result should I expect t 28 days when the strength of the remaining two cubes are at 99%