I got 10. Is that right?

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

Gabrielle's age is two times mikhail's age. the sum of their ages is 84. what is mikhail's age?

Ineed here’s a picture of the question