You ask 120 randomly chosen students to name their favorite ice cream flavor. there are 1800 students in the school. based on the results of the survey, predict the number of students in the school whose favorite ice cream flavor is chocolate chip.
vanilla= 36
chocolate=21
strawberry=24
chocolate chip=39

39/120 = x / 180039 choc. chip to 120 students = x choc. chip to 1800 students

now we cross multiply because this is a proportion
(120)(x) = (39)(1800)
120x = 70200
x = 70200/120
x = 585 <===

or u could do it this way as well...
39 out of 120 picked chocolate chip
39/120 = 0.325 = 32.5%so 32.5% chose chocolate chip.

32.5% of 1800 = 0.325(1800) = 585 <==