state how many significant figures are in 2500mg of magnesium

state how many significant figures are in 2500mg of magnesium
state how many significant figures are in 15.50mg of potassium
state how many significant are in 0.0052 grams of sodium chloride
The density of mercury is 1.36 x 104 kg/m3. Find the mass of a 4.00*10-4 m3 sample of mercury. A. 0.0343 kg B. 0.002 94 kg C. 2.94 kg D. 5.44 kg E. 6.29 kg