7.5. Grandfather is 31 times older than his grandson. In how many years will he be 7 times older than his grandson, given that the grandfather is more than 50 but less than 90 years old?
Answer: In 8 years.

Solution: Grandfather's age is divisible by 31. But the only such number greater than 50 and less than 90 is 62. Therefore, the grandfather is 62 years old, and the grandson is 2 years old. In x years, the grandfather will be x+62 years old, and the grandson will be x+2 years old. If at that time he will be 7 times older than the grandson, then x+62=7(x+2), from which x=8.

Comment: It is written that the grandfather's age is 62 years - 3 points.

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