If dexters room is 10 wide and 12'6" long and in his drawing is 4 wide is it true or false if I doulbe the quantity I doulbe the other?

Answer 1

But you didn't double.

but, yes, if the drawing is to scale, then you must multiply each dimension by the same scale factor

for yours:
10/12.5 = 4/x
10x = 50
x = 5

so a room 10 ft by 12.5 ft ----> 4 units by 5 units in your drawing.