Mllion peopledecade where t is measured in decades with t

Question

Question: U.S. CITIZENS 65 YEARS AND OLDER The number of U.S. citizens aged 65 yr and older from 1900 through 2050 is estimated to be growing at the rate of

R(t) = 0.063t² -  0.48t + 3.87      (0 ≤ t ≤ 15)

Million people/decade, where t is measured in decades, with t = 0 corresponding to 1900. Show that the average rate of growth of U.S. citizens aged 65 yr and older between 2000 and 2050 will be growing at twice the rate of that between 1950 and 2000.