SAT1000 - P904

If we define $S=\{a_{l_1},a_{l_2}, \cdots ,a_{l_n}\}$ as the $k$ th subset for set $E=\{a_1,a_2,\cdots,a_{10}\}$ , where $k=\displaystyle \sum_{i=1}^{n} 2^{l_{i}-1}$ .

Then what is the $211$ th subset for $E$ ?

How to submit:

Sort the indices of the elements of $S$ from smallest to largest and put them together. For example, if $S=\{a_1,a_2,a_3\}$ , then submit $123$ , and when $S=\{a_1,a_2,a_{10}\}$ , then submit $1210$ .

Have a look at my problem set: SAT 1000 problems