We know that F=m·a . For this particular problem, since 'm' is what we need to find, we can divide each side of the formula by 'm', and then it says m=F/a . Much more convenient.
Now all we have to do is write "100N" where it says "F", and write "20m/s²" where it says 'a'. Then the formula is m=(100N/20m/s²).
When you massage that out, it finally says "m=5 kg".
But I gotta tell ya, that's no soccer ball. On Earth, anything with 5 kg of mass weighs a hair more than 11 pounds ! You wouldn't want to play a game where you had to kick that thing or hit it with your head. 5 kg is more like a bowling ball.
