单选题
A certain fraction is equivalent to 
. If the numerator of the fraction is increased by 4 and the denominator is doubled, the new fraction is equivalent to 
. If the numerator of the fraction is increased by 4 and the denominator is doubled, the new fraction is equivalent to
【正确答案】
E
【答案解析】[解析] 设原分数的分子为x分母为y，则根据题意可得：
[*]
由方程(1)可得2y=5x，把它代入方程(2)可得[*]，y=15，所以原分数的分子与分母的和为x+y=21。
[*]
由方程(1)可得2y=5x，把它代入方程(2)可得[*]，y=15，所以原分数的分子与分母的和为x+y=21。