Question

The population of a city is 1,503,049. The land area of the city is 181.5 km².

Answer 1

Population density is population divided by area. So 

City: 1,503,049 people / 181.5 km^2 =  8281.262 = 8281 people/km^2

Watering hole. Same thing, population divide by area. Only gotcha is the area needs to be calculated. Using A = pi r^2, you get
3.14 * (34 km)^2 =  3629.84 km^2

32/3629.84 =  0.008816 = 0 antelopes/km

Chunk of mahogany wood.
Take the volume and multiply by the density. So
1728 cm^3 * 0.71 g/cm^3 =  1226.88 g

Gold bar.
Same thing. Volume multiplied by density. So
51.8 cm^3 * 19.3 g/cm^3 =  999.74 g

And finally, the sugar maple. This one is slightly trickier. You first need to calculate the volume of the block of wood. So divide the mass by the density.

82.8 g / 0.69 g/cm^3 =  120 cm^3

Now you know the length and width are 5x4 = 20 cm^2. So calculate the height by dividing the volume by 20 cm^2. So
120 cm^3 / 20 cm^2 = 6 cm.

Answer 2

1.) 8,281 people/km²
2.)
3.) 1226.88 grams
4.) 999.74 grams
5.) 6

I am not sure about number 2.