Emily writes down the largest two-digit prime such that each of its digits is prime.  Krish writes down the smallest two-digit prime such that each of its digits is prime.  Kirsten subtracts Krish\textquotesingle s number from Emily\textquotesingle s number.  What answer does Kirsten obtain? ． 
Overseas Competition->Knowledge Point->Number Theory Modules->Prime and Composite Numbers->Applying Special Prime Numbers
The prime digits are $$2$$, $$3$$, $$5$$ and $$7$$. So the largest two-digit integer whose digits are both prime is $$77$$. However, $$77$$ is not prime, nor is $$75$$, but $$73$$ is prime. So Emily writes down $$73$$. The smallest two-digit integer whose digits are both prime is $$22$$. However, $$22$$ is not prime, but $$23$$ is prime.  So Krish writes down $$23$$. Therefore the answer which Kirsten obtains is $$73 - 23 = 50$$. 