You just ask your questions in ask
X=t-yz and we are asked to solve for y so, subtract t from both sides
x-t=-yz divide both sides by -z
(x-t)/(-z)=y
y=(x-t)/(-z) multiply the numerator and denominator by -1
y=(t-x)/z
That's geometric sequence
T7 = T5*r^2 = T5+1920
r = 2, so
4*T5 = T5 + 1920
3T5 = 1920
T5 = 640
640 = 2^4 * 40
So, a=40 and the sequence is
40 80 160 320 640 1280 2560 5120 10240 20480 40960 ...
Note that 2560 = 640 + 1920
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One of the approaches:
Each couple can send only one "representative" to the committee. Let's see in how many ways we can choose 3 couples (as there should be 3 members) out of 4 to send only one "representatives" to the committee: 4C3=4.
But each of these 3 couples can send two persons (husband or wife): 2*2*2=2^3=8.
Total # of ways: 4C3*2^3=32.
hope it helped a bit
