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In 1974 a mother is 6 times as old as her daughter. The mother turned 50 in the year 2000. In what year was the mother double her daughter’s age?
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Since the mother was 50 years old in 2000, she was 50 - (2000-1974) = 50 - 26 = 24 years old in 1974.
Hence, the daughter was = 4 years old in 1974.
In "x" years after 1974, the daughter will be 4+x years old, while the mother will be 24+x years old.
We need to find "x" from the equation
2(4 + x) = 24+x.
8 + 2x = 24 + x,
x = 24 - 8 = 16.
1974 + 16 = 1990.
Answer. In 1990 the mother doubles her daughter’s age.
Check. In 1990 mother's age is/was 24 + 16 = 40 years.
In 1990 the daughter's age is/was 4 + 16 = 20 years. OK.