Why does the answer to question 3 give and answer to B-D starting with C rather than B? I thought we count the letter we start with?
Yes, counting
all the letters gives you the number. Counting the half steps
between the letters gives you the quality.
-----(Interval - letters - half steps - answer)
While he is explaining, first he writes out, then counts, the 3 letters involved-BCD. So you know it's some kind of third.
(Here you include the starting letter, the ending letter and all letters in between when counting.)
Then he writes the half steps between B and D and counts those. (Also 3)
(You
don't count the starting note here. You count the number of half steps up from there.)
Listen as he says "If we count half steps,
B, up a half step, I hope you remember, is C," then writes C.
He
is starting with B, but the half step up is C.
Then C up to C# is another half step.
Then C# up to D is another half step.
A total of 3 half steps to get from B up to D.
And the chart tells you that the answer based on having 3 half steps is a minor 3rd.
The notes C C# D are the half steps between B and D.
First he writes the notes between, and including, B & D (B C D), then he writes the half steps between B & D (C C# D).
Then he writes the answer.
The 3 letters in the interval tell you its a 3rd, then the 3 half steps in the interval tell you it's a minor 3rd.
For each example he writes out
all the letters involved, then the half steps in between, then the answer.