Answer: First train speed shall be 42 mi/hr
Second train speed shall be 84 mi/hr
Yes it is the valid answer, it gets annoying when almost everyone enters the wrong answer for a good laugh. Either way, hope this helps! Have a spectacular day.
Step-by-step explanation:
What is the 1st train's head start in miles?
Let +s+ = the speed of the 1st train in mi/hr
From 2PM to 4PM is 2 hrs
+d%5B1%5D+=+s%2A2+
---------------------
Let +d+ = distance the 2nd train travels until
it overtakes the 1st train
From 4PM to 6PM is 2 hrs
Start time when the 2nd train leaves
---------------------
Equation for 1st train:
(1) +d+-+2s+=+s%2A2+
Equation for 2nd train:
(2) +d+=+%28+s%2B42+%29%2A2+
--------------------
Substitute (2) into (1)
(1) +%28+s%2B42+%29%2A2+-+2s+=+s%2A2+
(1) +2s+%2B+84+-+2s+=+2s+
(1) +2s+=+84+
(1) +s+=+42+
and
+s%2B+42+=+42+%2B+42+
+s+%2B+42+=+84+
------------------
The 1st train's speed is 42 mi/hr
The 2nd train's speed is 84 mi/hr
---------------------------
check:
(2) +d+=+%28+s%2B42+%29%2A2+
(2) +d+=84%2A2+
(2) +d+=+168+
and
(1) +d+-+2s+=+s%2A2+
(1) +d+-+2%2A42+=+42%2A2+
(1) +d+=+84+%2B+84+
(1) +d+=+168+
converges if 
is monotonic and 
as 
. Here 
.
. Then 
, which is positive for all 
, so 
is monotonically increasing for 
must be a monotonically decreasing sequence over the same interval, and so must 
.