21 days before I had both the lessons together.
For the occurrence of two events at the same time we will find the least common multiple.
If I have piano lessons on every 7th day and tuba lessons on every 3rd day,
Least common multiple of 3 and 7 will be,
Multiples of 3 = 1 × 3
Multiples of 7 = 1 × 7
Least common multiple (LCM) of 3 and 7 = 3 × 7
= 21
Therefore, 21 days before I had both the lessons together.
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