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A fraction becomes 21тАЛ if 2 is added to the numerator and 5 is added to the denominator. It also becomes 31тАЛ if 2 is subtracted from both the numerator and denominator. The positive difference between the denominator and the numerator of the fraction is:
4
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3
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2
Set up the two linear equations in two variables from the given statements and solve simultaneously by cross-multiplication or elimination.
A fraction becomes 21тАЛ if 2 is added to numerator and 5 to denominator. It becomes 31тАЛ if 2 is subtracted from both.
y+5x+2тАЛ=21тАЛ,yтИТ2xтИТ2тАЛ=31тАЛ
Set up the two linear equations in two variables from the given statements and solve simultaneously by cross-multiplication or elimination.
Subtracting 2 from only one part instead of both numerator and denominator in the second condition.
Step 1: Formulate the equations from the given conditions
Let the fraction be yxтАЛ. According to the first condition, adding 2 to the numerator and 5 to the denominator gives 21тАЛ.
y+5x+2тАЛ=21тАЛтЯ╣2(x+2)=y+5тЯ╣2xтИТy=1
Step 2: Formulate the second equation
According to the second condition, subtracting 2 from both the numerator and the denominator gives 31тАЛ.
yтИТ2xтИТ2тАЛ=31тАЛтЯ╣3(xтИТ2)=yтИТ2тЯ╣3xтИТy=4
Step 3: Solve the simultaneous equations
Subtract the first equation (2xтИТy=1) from the second equation (3xтИТy=4) to find x.
(3xтИТy)тИТ(2xтИТy)=4тИТ1тЯ╣x=3
Step 4: Find the denominator y and calculate the difference
Substitute x=3 into 2xтИТy=1 to get y=5. The fraction is 53тАЛ. The positive difference between the denominator and numerator is 5тИТ3=2.
yтИТx=5тИТ3=2
D is correct because solving the equations yields numerator x=3 and denominator y=5, making the positive difference 5тИТ3=2.
Similar algebraic modeling questions appear frequently in age problems, speed-distance-time, and mixture ratios.