Apologies for being blunt, but 7.5kg dumbbells will do nothing.
Anyway, if you're just looking to lose weight then diet and cardio is what you need to look at.
What would be the weights needed for the dumbbells?
Well, how long is a piece of string? It completely depends on the person and how strong they are. Different movements will allow you to use more weight than others, depending on how many muscles are used and range of motion, etc. But the point is with any weight training movement you need to be lifting a weight which you can only handle for a certain amount of (relatively low) reps before your muscles fail (say, for example, 3 sets of 8 or 5 sets of 5), and there are simply very few movements with which 7.5kg would be anywhere near heavy enough to stimulate growth.
Also, the primary driver behind gaining muscle is linear progression (ie. increasing the weights you are lifting week on week, month on month, year on year). Therefore, even if in the highly unlikely situation that 7.5kg was personally heavy enough for you to lift and still stimulate growth, it wouldn't remain that way for very long. Therefore, aiming to use one size of weights for resistance training is utterly pointless - you need a selection of weights so that you can increase them as you progress.
It's very difficult to give you an idea of actual numbers, but considering you should be doing big compound movements such as squatting, benching, deadlifting and bent-over rows, and it's not unreasonable to estimate that an average beginner might bench anywhere from 30kg-50kg, squat 40kg-60kg, deadlift 60kg-80kg and row 30kg-50kg, then that might give you some idea of why getting 7.5kg dumbbells is completely pointless.
But as said, if you want to lose fat, then do cardio and look at your diet. Dumbbells, as with any kind of weight training, are primarily for gaining muscle mass.
post edited by CitizenKane - 2014/12/03 20:00:48