a. Given that for 1 adult, the amount of money collected is $6, then, for 90 adults the amount of money collected is 90x6 dollars.
Given that for 1 student, the amount of money collected is $2, then, for 50 students the amount of money collected is 50x2 dollars.
The total amount of money is the addition of these two amounts, that is,
Total = 90x6 + 50x2
b.
Total = 90x6 + 50x2
Total = 540 + 100
Total = 640 dollars