OG详解-OG2 数学1 Q18

Choice C is correct. Let  x  represent the number of children in a whale-watching tour group. Let  y  represent the number of adults in this group. Because it’s given that 21 people are in a group and the group consists of adults and children, it must be true that x + y = 21. Since the company’srevenue is 60  dollars per child, the total revenue from x  children in this group was  60x  dollars. Since the company’srevenue is  80  dollars per adult, the total revenue from  y  adults in this group was 80y dollars. Because it’s given that the total revenue for this group was 1,440 dollars, it must be true that  60x +80y = 1,440. The equations x + y = 21 and 60x +80y = 1,440 form a linear system of equations that can be solved to find the value of x , which represents the number of children in the group, using the elimination method. Multiplying both sides of the equation    x + y = 21 by 80 yields 80x +80y = 1,680. Subtracting 60x +80y = 1,440 from 80x +80y = 1,680 yields (80x +80y)- (60x +80y)= 1,680 -1,440, which is equivalent to 80x -60x +80y -80y = 240, or 20x = 240. Dividing both sides of this equation by  20 yields x = 12. Therefore, 12 people in the group were  children.
Choice A is incorrect and may result from conceptual or calculation errors. Choice B is incorrect. This is the number of adults in the group, not the number of children in the group. Choice D is incorrect and may result from conceptual or calculation errors.