Exercise 2 - solution

350 ml brewed at 40 mg/100 ml: 350 / 100 x 40 = 140 mg.
Two 30 ml espressos at 212 mg/100 ml: 60 / 100 x 212 = 127 mg (127.2).

Decaf (the exercise says "250 mg of beans' worth", meaning 250 mg of original caffeine):
97 percent removed leaves 250 x 0.03 = 7.5 mg.
99.9 percent removed leaves 250 x 0.001 = 0.25 mg.

Comparison: the brewed coffee holds slightly more caffeine than the two espressos; the EU standard leaves about 30 times less than the FDA standard (0.03 / 0.001 = 30).