Factor them
45=3*3*5
18=2*3*3
lcf=2*3*3*5=90
Morse code is essentially the same as binary. That is, there are two "digits", a dot or a dash.
There are 26 letters in the English alphabet. Clearly, we can't just use one dot or dash, since that could only encode 2 letters at the most. We can't use two symbols because that could only encode 4 letters at the most. Similarly, 3 symbols means

letters at most.
We have to select the smallest power of 2 that exceeds or is equal to 26. In this case,

, so we would have to use up to 5 symbols to encode each letter in the alphabet.
Answer:
10/3 hours
Explanation:
If Maria practices 5/6 hours a day, then she practices (5/6)(4) hours in 4 days.
(5/6)(4)=20/6
20/6 can be simplified to 10/3 hours.
I hope this helps!