The task is to generate the correct answer for a given math problem.
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Question: Problem: two trains of length 100 meters and 200 meters are 450 meters apart . they are moving towards each other on parallel tracks , at speeds of 90 km / h and 72 km / h . after how many seconds will the trains meet ?

Answer: 10


Question: Problem: the smallest value of n , for which n + 2 is not a prime number , is

Answer: 2,4


Question: Problem: a sum of money at simple interest amounts to $ 645 in 3 years and to $ 690 in 4 years . the sum is :

Answer:
$ 510