Hello there!

show your work and explain your answer. i don't just need the answer, i also want to know how to solve this kind of problems.

you : )

In the first answer, Paul James is kind of on the right track, 
but the problem is the CUBE of the way he describes it.

I thought about it for the past few hours, trying to come up with
the best way to explain it.  I think I can do it, and I hope it's clear.

Look at it this way:

In the REAL bridge, the volume of a little piece is

                  V = (Length) x (Width) x (Height) .

Whatever that volume is, it'll have 'V' units of steel in it,
and its weight will be 'V' units of weight.

Now, the engineers come along and build a scale model of the bridge.
The scale is  1:100 .
The "scale" always means the ratio of the LINEAR DIMENSIONS.
So that same little piece of the scale model will have

           length  =  1/100 of the REAL length
           width   =  1/100 of the REAL width
           height  =  1/100 of the REAL height .

The volume of the little piece will be

               v  =  (0.01 L) x (0.01 W) x (0.01 H)

  That's           (L · W · H) x (0.01 · 0.01 · 0.01)

                    =      V          x     (0.000001) .

                    =    one millionth of the volume of the real bridge !

The first answer said that if the scale is 1/100 , then the model
has  1/100  of the weight of the full-size bridge.  That's not true.

==>  If the scale is  1/100 , then the model has  (1/100)³
of the weight of the full-size bridge.  That's one millionth.

If the model weighs 50 Newtons, then the real full-size bridge
weighs 50 million Newtons.

Looking at it the other way ... If you scale the model down by 100, 
you scale the volume and weight down by (100)³  =  one million.

So the answer to ' b ' is: No!  The defense is wrong. It does not hold water !

At that scale, you could probably build the model out of cardboard and Kleenex,
and it would be stronger than the full-size structure built with steel !

Here's a general rule for scale models:

... You build a model to some SCALE.

... Each dimension of the model is (1/scale) the size of the original dimension.

... The AREA of the model or any part of it is (1/scale)² the size of
the original area.

... The VOLUME of the model or any part of it is (1/scale)³ the size of
the original volume.   That also goes for the weight, and the amount of
material you'll need to build them with.