There will be two parts, 1st for the answer, and 2nd for the extended version.
1. The answer is 1 3/4
2. Since he already had 4 1/2 and he needs 2 3/4 so he would be left with 1 3/4. That is due to the equation of "4 2/4 - 2 3/4 = ?", in this equation you would need to make the fraction 4 2/4 from a simplified fraction to an improper fraction. That is due to the 2/4 of the 4 1/2 since one half is smaller than 3/4. So you would take the 4 and make it a three, then you would add 4/4 to 2/4, resulting in 6/4, now you would have the equation of "3 6/4 - 2 3/4". So first you would subtract two from the three, leaving 1, and subtract three fourths from six fourths, leaving 3/4. You will put those results together to get, 1 3/4.