1. Right away, we can see that Z must equal 1, since it is the carry which becomes the first digit of the four digit number. Carries in two number addition can only be equal to 1. Knowing that Z=1:
Either Y+1= W or Y+1=10. It can’t be the latter, since that would make W=0, and thus, Y would have to be 1. Since Z is already equal to 1, this can’t be true. Therefore, Y+1= W.
Either Y+W=11 or Y+W=1. The first equation must be true since the other equation would make Y and W equal to 0 and 1. Z is already equal to 0. So now we know that Y+W=11.
Substitute the first equation in for W in the second equation: Y+Y+1=11. Isolate and solve for Y. This makes Y=5. When this is substituted back into the first equation, we find that W=6.
Plug W, Y, and Z back into the first equation, and you find that X=9.
2. The first thing to notice about this problem is that when the first number is multiplied by the second digit of the second number, the product is the first number. Thus, the second digit of the second number, X, must be 1 (the multiplicative identity).
Next, form an equation from the problem using place, keeping place value in mind. Simplify and solve for Z.
(10+Y)(10Y)=100Y+10Z
100Y+10Y(Y)=100Y+10Z
10Y(Y)=10Z
Y(Y)=Z
Next, find all the possible combinations of Y and Z that would make this equation true within the problem. Y can’t be 0 or 1, since that would make Z=Y. Both 2 and 3 are possible values of Y, corresponding to values of 4 and 9 respectively for Z. Y can’t be greater than or equal to 4, because that would make Z a 2-digit number. Thus, Y=2 or 3 and Z=4 or 9.
Test out both combinations in the problem. If 2 and 4 are substituted, W=2, since Y is already 2, this combination doesn’t work. Thus, X=1, Y=3, Z=9, W=4, and V=0.
3. X=1, Y=0. This is immediately evident from the addition of the last digits in both numbers. Since X+X leaves Y and possibly a carry, X cannot equal 0 if X is distinct from Y. If X was 0, then X+X would be 0, since 0+0=0. However, that would make Y=0, and X and Y are not the same digit, so X must be 1.
4.
Step 1: James and Alyssa to the library (2min)
Step 2: James back to study hall (1min)
Step 3: Josh and Christine to the library (8min)
Step 4: Alyssa back to study hall (2min)
Step 5: James and Alyssa to the library (2min)
Total time: 15min.
Chances of four Pingry students cooperating long enough to figure out this algorithm and implement it within one period: Alarmingly low
5.
5 terms: 3 searches.
Search 1: Find 3, eliminate 3 terms, left with 2 terms. Search 2: Find 1, eliminate 1 term, left with 1 term. Search 3: Find term.
50 terms: 6 searches
Search 1: Eliminate 25 terms, left with 25. Search 2: Eliminate 13 terms, left with 12. Search 3: Eliminate 6 terms, left with 6. Search 4: Eliminate 3 terms, left with 3. Search 5: Eliminate 2 terms, left with 1. Search 6: Find term.
1,000 terms: 10 searches.
Search 1: Eliminate 500 terms, left with 500. Search 2: Eliminate 250 terms, left with 250. Search 3: Eliminate 125 terms, left with 125. Search 4: Eliminate 63 terms, left with 62. Search 5: Eliminate 31 terms, left with 31. Search 6: Eliminate 16 terms, left with 15. Search 7: Eliminate 8 terms, left with 7. Search 8: Eliminate 4 terms, left with 3. Search 9: Eliminate 2 terms, left with 1. Search 10: Find term.
100,000 terms: 17 searches.
Search 1: Eliminate 50,000 terms, left with 50,000. Search 2: Eliminate 25,000 terms, left with 25,000. Search 3: Eliminate 12,500 terms, left with 12,500. Search 4: Eliminate 6,250 terms, left with 6,250. Search 5: Eliminate 3,125 terms, left with 3,125. Search 6: Eliminate 1,563 terms, left with 1,562. Search 7: Eliminate 781 terms, left with 781. Search 8: Eliminate 391 terms, left with 390. Search 9: Eliminate 195 terms, left with 195. Search 10: Eliminate 98 terms, left with 97. Search 11: Eliminate 49 terms, left with 48. Search 12: Eliminate 24 terms, left with 24. Search 13: Eliminate 12 terms, left with 12. Search 14: Eliminate 6 terms, left with 6. Search 15: Eliminate 3 terms, left with 3. Search 16: Eliminate 2 terms, left with 1. Search 17: Find term.
Finding Josh: 1 search. Josh is the fifth of nine students in alphabetical order (Christine, Derek, Freddy, James, Josh, Rahul, Sharanya, Wenrui, Zach). Binary search looks at the middle term first, and since Josh is the middle term, which is the fifth term in this case, it would only take one search to find him.
6. To sort the first five and last five elements of a list, begin with the first number in the list. Search for the lowest number in the list to the right and switch it with the first number. The move on to the second, third, fourth, and fifth elements of the list and do the same. Then, move to the other end of the list and search for the highest number to the left. Switch that with the last element of the list. Then, do the same for the second-, third-, fourth-, and fifth-to-last numbers. The first five numbers should be the lowest of the list in order, and the last five numbers should be the highest of the list in order.
Confused? Don’t be. This is the Chrono-Synclastic Infundibulum. Everyone is always right.
Have a nice day.
Sharanya out.