Bunuel
Route 1: City Centre – Market Square – Songbird Park – City Centre (3 miles)
Route 2: City Centre – Laurel Lane – Market Square – City Centre (3 miles)
Route 3: City Centre – City Hall – Laurel Lane – City Centre (3 miles)
Route 4: City Centre – City Hall – Market Square – Laurel Lane – City Centre (4 miles)
Route 5: City Centre – Songbird Park – Market Square – Laurel Lane – City Centre (4 miles)
A cross-country runner trains by running three predetermined routes every day. Each day, her total distance travelled is 10 miles. She does not run any of the routes more than two days in a row and never avoids a route two days in a row.
For the
City Centre, select the number of times the runner will run towards the city centre over the span of any 3-day period. For
Laurel Lane, select the number of times the runner will run towards Laurel Lane over the span of any 3-day period. Make only two selections, one in each column.
The only possible combination of route lengths out of the given lengths that sum up to 10 miles is 4 miles + 3 miles + 3 miles. Thus we will consider all the possible cases of routes in a day. Keeping in mind that the runner avoids running a route *more than two days in a row* and avoids avoiding a route *two days in a row*.
4 miles: It can travel either Route 4 or Route 5, each possible combination of 3 miles + 3 miles will have the two choices of Route 4 or Route 5 for us to combine with.
3 miles + 3 miles:
Say following are the possible route choices made on Day 1: R1 and R2 OR R1 and R3 OR R2 and R3. Following are the possible combinations for Day 1- Day 2- Day 3 (with each combination able to be paired with R4 or R5).
1. R1R2 - R1R3 - R2R3 repeat OR R1R2 - R2R3 - R1R3 repeat
2. R1R3 - R1R2 - R2R3 repeat OR R1R3 - R2R3 - R1R2 repeat
3. R2R3 - R1R2 - R1R3 repeat OR R2R3 - R1R3 - R1R2 repeat
Now that we've found all possible combination, let's answer.
Number of times city center will be 'ran towards' in a three day period.
Since city centre is common to all routes, we can take any possible combination and count, say R4R1R2 — R5R1R3 — R4R2R3, in a day city centre is started from, which doesn't count, DAY 1, reaching R1 (1) reaching R2 (2), now at city centre, the route changes to R5, DAY 2, reaching R1 (3), reaching R3 (4), now at city centre, the route changes to R4, DAY 3, reaching R2 (5), reaching R3 (6). Thus City Centre is ran towards 6 times.
Number of times laurel lane will be ran towards in a three day period.
Again choosing the combination R4R1R2 — R5R1R3 — R4R2R3. We have DAY 1, laurel lane, once in R4, once in R2 (2), DAY 2, once in R5, once in R3 (2), DAY 3, once in R4, once in R3 and once in R2 (3), thus a total of 7 times.
Posted from my mobile device