Segment drawn on scale having end points O (0,0) and A
is a line segment.
O A=
![\sqrt{[(3)/(4) -0]^(2) +[(9)/(10) -0]^(2)\\\\](https://img.qammunity.org/2019/formulas/mathematics/high-school/52xp86j34f46zvcydzvc8xgp31hd8lgjwk.png)
O A =

=

=

Now , the same segment actual structure having end points O'(0,0) and B (30,36) is also a line segment.
O'B=

=

=

= 2√549
[Cancelling √549 from numerator and denominator]
So, Actual length = 40 × Length on scale