The scale model of a skyscraper being built is 4.2 feet tall.
a. When it is finished, the skyscraper will be 525 meters tall. What
scale was used to make the model?

The scale that was used in the model can be calculated by first converting the height of the model from feet to meter in order to have consistency in the units.
height of model = (4.2 ft)(1 m/3.28 ft) = 1.28 m
Since it is said that this corresponds to 525 m actual height of the skyscraper, the scale used was:
1.28 m: 525 m
or simply,
32:13125

For this case we must take into account the following transformation of units:

1 meter = 3.28 feet

Applying the transformation of units we have:

$$(4.2feet)(\frac{1}{3.28}\frac{m}{feet}) = 1.28 m$$

Thus, the scale used for the model is given by:

1.28:525