So I can throw my "worksheet" away(!):

Code:

A..........a=700.............>A,B<......................................B
B<............................B,A......................................>A
. .
B.................................................>B,A<...b=400.........A

Make the 2 boats A and B ; and let a =700, b = 400; let d = distance

A:B "distance travelled" ratios:

From 1st meeting: a / (d - a)

From 2nd meeting: (d + b) / (2d - b)

a / (d - a) = (d + b) / (2d - b)

Leads to: d = 3a - b

So distance = 3(700) - 400 = 1700

Speeds don't matter; buy they'll be in same ratios, of course.