How many days are between K th day of the K th week to 2K th day of the 2K th week

Number of days in K weeks is 7K hence total number of days is 7K + K = 8k
Similarly number of days in 2Kth day of the 2Kth week is 2k x 7 + 2k = 16k
Required number of days is 16K - 8K = 8K or number of days must be a multiple of 8.