In one year, one dollar ($1) could be exchanged for one euro and twenty cents (€1.20). 

a. Derive a formula to exchange dollars into euros.

Let d be the number of dollars.

Let e be the number of euros.

From the information in the question, e=1.2 when d=1. So the formula is:

e = 1.2d

b.  Use your formula to work out how many euros you would receive for $250.

Substitute d = 250 into the formula:

e = 1.2 × 250

e = 300

You would receive €300 in exchange for $250.