17.Consider a function which， for a given whole number n， returns the number of ones required when writing out all numbers between 0 and n. For example， f（13）=6. Notice that f（1）=1. What is the next largest n such that f（n）=n？
18.What's the coolest hack you've ever written？
19.'Tis known in refined company， that choosing K things out of N can be done in ways as many as choosing N minus K from N： I pick K，you the remaining.
Find though a cooler bijection， where you show a knack uncanny， of making your choices contain all K of mine. Oh， for pedantry： let K be no more than half N.
20.What number comes next in the sequence：10， 9， 60， 90， 70， 66，？
A）96 B） 1000000000000000000000000000000000 0000000000000000000000000000000000 000000000000000000000000000000000 C） Either of the above D） None of the above
21.In 29 words or fewer， describe what you would strive to accomplish if you worked at Google Labs.