Bill's statement does not make sense because it is incorrect:
7/12 customers bought pretzels
3/10 OF those customers bought low salt pretzels (x)
3/10 of 7/12 = x
(the word "OF" normally refers to multiplication and he variable x represents low salt pretzels)
3/10 times 7/12 = x
21/120 = x
Simplify: 7/40
To be correct, Bill would have to say that 7/40 of the customers bought low salt pretzels, but instead he had said 7/30.