Concrete Cube Compressive Test

M20-for 7 days it should be 67% and for 28 days it should be 100%...formula for cube testing:
ex: m20
size of cube surface area is 15*15 =225/10=22.5

for m20 7 days 20 * 22.5 * 67% = 301.5 kn.
28 days 20 * 22.5 = 450 kn.

We have made a rcc cube test of M25 and the report of cube test says that it has comp. strength of 7.33 (N/mm2) for the first cube for the second cube it is 12.08 (N/mm2) and for the third cube it says 9 (N/mm2) now we are not able to understand that is this good strength of not. and all this cube where having 30 days of curing. one more this what is IS 456 and IS 516

Ex for M 25.

Area of cube 150*150 mm=22500 mm2 so we need to n per mm2 so we get 22500/1000=22.5N/mm2

so M25 for 7 days load/area=so load should be above 16 n/mm2 = 67% Strength

M25 for 25 days load/area =so load should be above 563 KN 100% strength.