Inner radius: 3/2 = 1.5 (decimeter)
Oil drum volume: 3.14×1.5×1.5× 4 = 28.26 (cubic decimeter) = 28.26 (liter)
0.7× 28.26 = 19.782 (kg)
19.782 (kg) < 20kg,
So Xiao Qiang can fill this oil drum with 20 kilograms of gasoline.