1.
1 out of 10 of 500 customers is 50 customers surveyed. So there must be 20 who choose 3-D.
Answer: 20
2. The every 10th part is fine; it's the Sunday afternoon that makes this a representative sample of folks who show up on Sunday, but maybe one for entire week. Presumably we get more kids on Sunday afternoon than a weekday evening, for example.
Answer: No
3. 100:25 = 12:x so x = 25*12/100 = 3
Answer: 3
Real surveys get answers from who they reach and then try to correct for the undersampling or oversampling of some group after the data's been collected.
4. weekday:total = 40:40+80 = 400:x
x = 400(40+80)/40 = 1200
Answer: 1200