Answer: 28 days
Explanation:
Given : You have piano lessons every seventh day and cooking lessons every fourth day.
The number of days it takes to have both lessons on the same day will be the least common multiple of 7 and 4.
Using prime factorization,

LCM(7,4)=

Hence, if today you have both lessons then after 28 days you will have both lessons on the same day again.