Find the missing term?

Question

          /\
         /  \
      5 / 12 \ 6
       /______\
           4       

          /\
         /  \
      6 / 21 \ 7
       /______\
           5      

          /\
         /  \
      4 / ?  \ 8
       /______\
           10

The question is to find the missing term.

Answers - a) 22 b) 30 c) 32 d) None of the above

Solution given in the book -

Clearly we have: (5X6X4)/10 = 12

              (6X7X5)/10 = 21

              (4X8X10)/10 = 32

Hence the answer is 32

My confusion in this solution - Why they came up with 10 (in bold)? They could come up with any other numbers for example 5 or 20 or anything else. Why 10?

My solution -

Clearly we have (5X4 - 6) - 2 = 12

            (6X5 - 7) - 2 = 21

            (10X4 - 8) - 2 = 30

So my answer is 30

My question - I didn't find anything wrong with my answer. So why will I take my book's solution only? But surprisingly with both approach the answers are not the same. My answer is 30 and the solution book gave has 32 as an answer. Both 30 and 32 is among the choice of answers in the question.?

Answer 1

Thanks to Ross Millikan and DonAntonio for the discussion on this. And I got the answer. Here it is.

As for the answer I suggested -

This is not an universal answer. What I mean to say is that if in case of the 2nd triangle the middle number was changed to 35 for example, my suggested formula didn't work. In that case I had to use numbers from different sides of the triangle that shouldn't be identical for the case of other triangles.

But the solution of the book is universal, as it is.

I got the answer now.

Even saying so, the question itself is ambiguous.