In 2007, U.S. consumption of renewable energy was 6.8 quadrillion Btu (or 6.8 × 1015 Btu). Since the 1960s, consumption has been growing at a rate (in quadrillion Btu's per year) given by
f'{t)= 0.004r + 0.062
where this years after 1960. Find f(t)and estimate U.S. consumption of renewable energy in 2020.