Just some given and reason statements, please help

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

The difference of three and the reciprocal of a number is equal to the quotient of 20 and the number. find the number.

Zack and tia played chess for 50 min they put the chessboard away at 11: 20 when did they start