Initially, the trains were 240 km apart.
The fly is flying at the speed of 90 km/h and covers 180 km. Hence, the fly flies for 2 hours after trains started.
It's obvious that trains met 2 hours after they started travelling towards each other. Also, trains were travelling at the speed of 60 km/h. So, each train traveled 120 km before they met.
Hence, the trains were 240 km apart initially.
What is the minimum number of numbers needed to form every number from 1 to 7,000? Example: To form 4884, you would need 2 4s & 2 8s. 4822 requires a 4,
a 8, & 2 2s, but you would not count the numbers again that you had already counted from making 4884.