#### Answer

(a) Grandson.
(b) Incorrect reasoning.

#### Work Step by Step

(a)
We need to evaluate:
$9+15\div3$
Frank used the wrong order of operations:
$9+15\div3=(9+15)\div 3=24\div3=8$ (incorrect)
If we follow the proper order of operations, we need to divide first, then add. In other words, we need to:
$9+(15\div3)=9+5=14$
So the grandson's answer is correct.
(b)
The grandson's answer is correct, but his reasoning is incorrect. In this particular case, going right to left works, but that rule will not work in general.