How do you calculate the degrees of freedom for a t distribution what happens to the shape?

Let r = 3. what is the value of r + 58?

For the rule of 78, for a 12-month period, the last term in the sequence is 12 and the series sums to 78. for an 10 month period, the last term is and the series sum is . for a 15 month period, the last term is and the series sum is . for a 20 month period, the last term is and the series sum is

In a column proof the right column states your reasons