find derivation trees for CFG I need to draw the derivation tree for $1-2-(3-4)*5*6$ from the grammar below.
 I want to know how many possible derivation trees are there from this grammar.

$$\begin{align}V_n&amp;=\{expr,term,factor,number\}\\
V_t&amp;= \{(,),-,*,0...9\}\\
P&amp;=\left \{
\begin{aligned}
expr&amp;\to expr-expr\;\mid\;term\\
term&amp;\to term*factor\;\mid\;factor\\
factor&amp;\to number \;\mid\; (expr) \\
number&amp;\to 0\mid1\mid2\mid3\mid4\mid5\mid6\mid7\mid8\mid9
\end{aligned}
\right \}\\
S&amp;=expr
\end{align}
$$

The possibilities that I can find are: 

$$(1-2)-(((3-4)*5)*6)\\
1-(2-( ((3-4)*5)  *6))$$

Are there other possibilities?
No, there are no other possibilities as far as I can see.

The ambiguity comes from the production rule $expr \rightarrow expr - expr$. This rule allows you to derive the expression in two different ways:


Derive $1$ from the left $expr$ and the rest from the right $expr$
Derive $1-2$ from the left $expr$ and the rest from the right $expr$


Some ideas that might help.

The precedences look ok to me; the lower the precedence the closer the operator should be to the root (in this case $expr$).
- has the lowest (derived from $expr$), * has a higher precedence (derived from a production after $expr$).
And the parentheses (...) have the highest (derived from a production after $term$).

The $term$ production rule is left associative, which helps avoid the ambiguity of associativity.

The $expr$ production rule is the problem as mentioned above. A solution to that would be similar to what you see for the $term$ production rule. Make it left associative.