2/(2/3)+3/4/(4/5)=

The answer key explains it like this:
2/(2/3)+3/(3/4)+4/(4/5)=2x3/2+3x4/3+4x5/4= 3+4+5=12
*I do not understand how the answer is 12.

If 2/5 of (2/3)/(5/6) is divided by 3/10, what is the resulting value?
The answer key explains it like this:

2/5((2/3)/(5/6))/3/10=2/5x2/3x6/5x10/3=8/15

I do not understAnd why the answer is 8/15.

2 answers

Well, your original line of text is not the same as the answer key's explanation.

I assume you get 3+4+5=12
Now, 2/(2/3) = 2*(3/2) = 2

when you divide by a fraction, it's the same as multiplying by its reciprocal.

In fact, when you divide by anything, it's the same as multiplying by its reciprocal. 2/3 = 2(1/3)
1. 2/(2/3) + 3/4/(4/5).
2/(2/3) + (3/4)/(4/5) =
2*(3/2) + (3/4)*(5/4) =
6/2 + 15/16 =
48/16 + 15/16 = 63/16

2. (2/5)(2/3)/(5/6)/(3/10) =
(2/5)(2/3)(6/5)(10/3) = 240/225 = 48/45 = 16/15.
The answer cannot be less than one.