Give one example of when you would use the friendly numbers strategy to subtract

This race was first held in 1953 on august 16th. every decade the race finishes with a festival. how many years is this?

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

Which one ? is the answer and how to find this answer