## College Algebra (6th Edition)

a. The lower limit uses the first formula. For age 20, substitute 20 for each "a" in the formula: $$H=\frac{7}{10}(220-a)$$ $$H=\frac{7}{10}(220-20)$$ $$H=\frac{7}{10}(200)$$ By writing the 200 as a fraction, you can cross reduce: $$H=\frac{7}{10}(\frac{200}{1})$$ $$H=\frac{7}{1}(\frac{20}{1})$$ $$H=7\times20$$ $$H=140$$ b. The upper limit uses the second formula. For age 20, substitute 20 for each "a" in the formula: $$H=\frac{4}{5}(220-a)$$ $$H=\frac{4}{5}(220-20)$$ $$H=\frac{4}{5}(200)$$ By writing the 200 as a fraction, you can cross reduce: $$H=\frac{4}{5}(\frac{200}{1})$$ $$H=\frac{4}{1}(\frac{40}{1})$$ $$H=4\times40$$ $$H=160$$