Please help 1/2P-10=15

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

What are the equivalent ratios for 24/2= /3= /5.5=108/ = /15

If chord ab is congruent to chord cd, then what must be true about ef and eg?