Explanation:
which one of the 3 categories represent the 6m ?
the 7 ? the 3 ? or the 5 ?
I suspect it is the 3 (the middle), because it is the only one working with 6 using only integer numbers.
you really have to be more careful about putting all relevant information here for a question.
so,
in general, we see that a "whole" set of distance types has 7 + 3 + 5 = 15 parts (in our case equally long distance segments).
the length of the first type (whatever it is) is 7 parts of the whole 15 parts. = 7/15
the length of the second type is 3 parts of the whole 15 parts (and we suspect this type to be 6m). = 3/15
and the length of the third type is 5 parts of the whole 15 parts. = 5/15
since we now know (or rather assume) that 6m = 3 parts, we can then calculate that 1 part of the 15 is 6/3=2m.
aha !
so then 7 parts are 2×7 = 14m.
and 5 parts are 2×5 = 10m
I guess that's what you needed to find, right ?
just to practice : what would it be, if 6m stands for the 7 ?
we still have the 15 parts of the whole.
but now 7 parts stand for 6m.
1 part is then 6/7m.
3 parts would then be 3×6/7 = 18/7m
5 parts would be 5×6/7 = 30/7m
you see now, why I think the 6m were meant for the 3 part category ?