“I’ve always been 45 years older than your Dad,” said Grandma to young Trickle, “But now the two digits in my age, both prime, are the reverse of the two in your Dad’s age.” How old is Grandma?
Answer
Grandma was 72 years old. Let the two digits be A and B. Then 10A+B-10B-A=45 so 9(A-B)=45. This gives A-B=5. Of the pairs 5,0; 6,1; 7,2; 8,3; 9,4 only 7,2 are both prime numbers.
