The following table gives the the approximate amount of emissions, $E$, of nitrogen oxides in millions of metric tons per year in the US. Let $t$ be the number of years since 1940 and $E=f(t)$.

(a) Using left endpoints, estimate the integral:
$\displaystyle \int_0^{50} f(t) \, dt \approx$

(b) Using right endpoints, estimate the integral:
$\displaystyle \int_0^{50} f(t) \, dt \approx$

Be sure that you know what the units of your answer are, and what its meaning is!

(Original data from the Statistical Abstract of the US, 1992)