Question 975838
One sample proportion test looking for 
z=(.35-.30)/sqrt[(0.30)(0.70)/650];;;; denominator is .

z=0.05/0.0179
=2.793

What is this?  It is the probability that more than 35% will buy the desert given that 30% is the base probability.
It is 0.0026.

With a large sample size, the likelihood of this occurring by chance will be small.  Note, the calculator gives a different result, because you are using 228 students for 35%, not the exact 227.5.