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If every student who was in the eleventh grade in 1996 and who used a computer in school at least once a week used a computer at least once a week in 1997, and if 20% of those students who were in the eleventh grade in 1996 and who did not use a computer in school at least once a week, did use a computer in school once a week in 1997, then what percent of twelfth-graders in 1997 used a computer in school at least once a week
A．(A) 40%
B．(B) 50%
C．(C) 60%
D．(D) 70%
E．(E) 80%
参考答案：C
解析：In 1996, 50% of eleventh-graders used com-puters in school at least once a week, and 50% did not. Of the 50% that did not use com-puters in school at least once a week, 20% of them did use them the following year: 20% of 50% = .20 × .50 = .10 = 10%. So, in t997, 60% (50% + 10%) of the twelfth-graders used computers in school at least once a week.