pop = {{1900, 1650}, {1910, 1750}, {1920, 1860}, {1930, 2070}, {1940, 
          2300}, {1950, 2520}, {1960, 3020}, {1970, 3700}, {1980, 
          4450}, {1990, 5300}, {1996, 5770}}

TableForm[pop]

listplot  =  ListPlot[pop]

f[x_]  = Fit[pop, {1, x, x^2, x^3}, x]

modplot  = Plot[f[x], {x, 1900, 2000}]

Show[listplot, modplot]

f[1925]