Answers: A and 64
for the first question the series would look something like
(7,9,-9,-7),(7,9,-9,-7)… so its repeating after every 4 terms the 80th term is -7 (4 terms repeated 20 times) so 79th term is -9
for second question
There may be much better way to solve but I solved it like this so its a decreasing series with a constant decrease so I chose first term as 7 and then my series looked like
7,6,5,4,3,2,1 now m2+m4+m6=12 but question has 48 so i multiplied every term by 4 and found m1+m3+m5+m7 and got 64
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for the second one better approach would be like M(n)=M(1)+(n-1)d
so m2+m4+m6=m1+3d=16
and m1+m3+m5+m7=4(m1+3d) so its 16*4 thus the answer is 64
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For second one also,I think this solution is much quicker. Great job!