In the same section, there’s a link to a Capture the Flag. This CTF consists of challenges in 5 different categories and for each challenge there’s a score and the estimated time (in minutes) for solving it. R-Boy is really good in WEB, so he can solve those challenges in half the estimated time, but he needs twice the time to solve BINARY challenges. The CTF lasts only 24 hours. Can you help R-Boy choose which challenges to solve to achieve the best score possible – and win the prize? The IDs of the challenges to choose, sorted by ETA descending, will reveal you the prize

The challenge provide us a .csv file with the list of challenge with the ids, categories, scores and ETAs:

ChallengeID	Category	Score	ETA
N		binary		100	35
p		binary		200	55
q		binary		300	85
I		binary		500	140
j		binary		1000	125
V		coding		100	25
U		coding		200	45
i		coding		300	95
B		coding		500	150
0		coding		1000	180
6		crypto		100	35
R		crypto		200	40
F		crypto		300	60
e		crypto		500	110
f		crypto		1000	200
x		miscellaneous	100	20
J		miscellaneous	200	5
A		miscellaneous	300	100
M		miscellaneous	500	80
P		miscellaneous	1000	160
t		web		100	30
Z		web		200	70
8		web		300	50
Q		web		500	100
D		web		1000	90

The problem is a simple 0/1 knapsack problem, where we have a knapsack of capacity 1440 (minutes). We only have to pay attention as we have to consider only half of the time for the web challenges and double of the time for the binary challenges.

To solve the problem we can simple use an online solver like this

The challenge that provide us the best solution are (already sorted):

ChallengeID	Category	Score	ETA
f		crypto		1000	200	
0		coding		1000	180
P		miscellaneous	1000	160
j		binary		1000	125
e		crypto		500	110
Q		web		500	100
i		coding		300	95
D		web		1000	90
M		miscellaneous	500	80
Z		web		200	70
F		crypto		300	60
8		web		300	50
U		coding		200	45
R		crypto		200	40
t		web		100	30
V		coding		100	25
x		miscellaneous	100	20
J		miscellaneous	200	5

The list of choosen IDs, sorted by ETA desc, are f0PjeQiDMZF8URtVxJ so the flag is {FLG:f0PjeQiDMZF8URtVxJ}