Let's convert everything to the same units here. Half of an hour is 30 minutes. Now, set up a proportion that states he can finish 1 assignment in 30 minutes and x assignments in 20 minutes:
[tex] \frac{1}{30} = \frac{x}{20} [/tex]
Use cross multiplication:
[tex]20 = 30x \\ x = \frac{20}{30} = \frac{2}{3}[/tex]
Jeff can finish 2/3 of his assignment, which is about 67%.
