单选题 In 1950 Roberto was four times as old as Juan. In 1955 Roberto was three times as old as Juan. How old was Roberto when Juan was born?
  • (A) 5
  • (B)10
  • (C) 20
  • (D) 30
  • (E) 40


【正确答案】 D
【答案解析】Make a table to determine Roberto's and Juan's ages. Let x represent Juan's age in 1950, and fill in the table as shown./r/n /r/n /r/n /r/n /r/n /r/n /r/n /r/n /r/n /r/n /r/n /r/n /r/n /r/n /r/n /r/n
1950
1955
Roberto
4x
4x+5
Juan
x
x+5
In 1955, Roberto was 3 times as old as Juan,
so
[*]
Therefore, in 1950, Juan was 10 and Roberto was 40. Because Roberto is 30 years older than Juan, Roberto was 30 when Juan was born.