Let the number of 1-dollar notes with Mr. Smith be ‘p’ and the number of 1-euro notes be ‘q’.
Since he has 30 notes, p + q = 30
Total value of the dollar notes with Mr. Smith = p * 1 = p dollars.
Since 1 dollar = 40 euros, total value of the dollar notes with Mr. Smith in terms of Euros = 40p euros
Total value of the euro notes with Mr. Smith = q * 1 = q euros.
Total euros with Mr. Smith = 40p + q.
If the euros were dollars and the dollars were euros,
Number of 1-dollar notes = q and number of 1-euro notes = p
Total value of 1-dollar notes with Mr. Smith = q * 1;
Since 1 dollar = 40 euros, total value of the dollar notes with Mr. Smith in terms of Euros = 40q euros
Total value of the 1-euro notes with Mr. Smith = p * 1 = p euros.
Total euros with Mr. Smith = 40q + p.
In the second case, Mr. Smith would have 78 euros more than what he had in the first case. Therefore,
40q + p = 40p + q + 78
Simplifying, 39q – 39p = 78 or q – p = 2.
Solving the two equations in p and q, p = 14 and q = 16
Number of 1-dollar notes with Mr. Smith = 14; therefore, number of dollars he has = 14.
The correct answer option is C.