How many days are between Kth day of the Kth week to 2Kth day of the 2Kth 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.