3 1/2 seconds can also be written as 35/10 seconds. If in 1/10 of a second, the train goes 70ft, then simply times 70 by 35. This equals 2450. This particular train will travel 2450 feet in 3 1/2 seconds.
Well if it travels 70 feet per 1/10 of a second, you would have to figure out how many tenths of a second are in a minute (600). The multiply that by 3 1/2 minutes (2100). Then multiply that by 70 feet and you will get your answer of 147,00 feet.