45+(-30)-90 the answer of that mathematics sum

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

Empty box + box + empty box fill in the box is equal to 30 how

Can someone me with the property questions for 12 13 and 14 plz