When the sun warms a swimming pool, heat is transferred through ?

Well, it can't be conduction or convection ... both of those need some material stuff along the way to move the heat from one place to another.

The only other way there is to move heat is by 'radiation' .  That's the method that doesn't meed any material 'medium' along the way.  The energy gets carried through empty space by electromagnetic waves.  

answer: 2×10^-2kg

explanation: density= mass per volume

vol=mass/density

converting to s.i units we have;

3÷1000kg = 0.003

1.5÷100m = 0.15

0.003/0.15 = 0.02kg