Answer: the greatest is 30% of 30


Finding the values
50% of 10 = 10$\div$2 = 5                           

40% of 20 = 10$-$2 = 8 

30% of 30 = 3$\times$3 = 9        

20% of 40 = 4 + 4 = 8

10% of 50 = 50$\div$10 = 5

So 30% of 30 is the greatest


Using commutativity and strategy
$50\% \text{ of } 10 = \frac{50}{100}\times10 = \frac{10\times50}{100} = \frac{10}{100}\times50 = 10\%\text { of }50$

Similarly, $40\% \text{ of }20 = 20\%\text{ of }40$

So $30\%\text{ of }30$ is the only one without a partner - so it must be the greatest, because there is only one answer