```
#!/bin/sh --

expression_to_evaluate='12 + 3'

printf '%s\n' "Arithemtic Expansion: $(( $expression_to_evaluate ))"

printf '%s' 'bc: '
printf '%s\n' "$expression_to_evaluate" | bc

printf '%s' 'awk: '
awk -v expression_to_evaluate="$expression_to_evaluate" -- 'BEGIN{printf "%d\n", expression_to_evaluate}'

```

Output:

```
Arithemtic Expansion: 15
bc: 15
awk: 12

```

`awk` is returning 12 instead of 15. How can I feed a string to `awk` for the purpose of mathematical calculation?