First solve for the density of the substance (in this example, I will denote cubic meters simply as m and cubic centimeters simply as cm to avoid confustion):

D = M/V = 800kg/1m

Convert this density to g/m:
(800kg/1m)*(1,000g/kg) = (800,000g/m)

Find out how much volume in terms of m is in 1g:
(800,000g/m) = (1g/x)
Cross multiply (the units g and m will cancel out):
1 = 800,000*x
x = 1/800,000

The above equation tells you that 1 gram of substance has a volume of (1/800,000)m. Now we want to convert this to cm:

(1/800,000)m*(1,000,000cm/1m) = 10/8 = 1.25

Hope that helps.