If a_1=5a 1
​
=5 and a_n=na_{n-1}-4a
n
​
=na
n−1
​
−4 then find the value of a_3a
3
​
.

Because it is the very esays way to learn

In order to find the price per bar, we divide the price by the amount of bars.   for the first one: 15.37/10 = $1.54 per barthe second package: 15.35/12 = $1.28 per bar.the 10-pack costs $1.54 per bar and the 12-pack costs $1.28 per bar.   the 12-pack has the better price per bar.now, let's look at the price per ounce.   we do this in a similar way.   we find the total amount of ounces in the package, and divide the price by the number of ounces.in the first package, we multiply 10*2.1=21.   we have 21 ounces in the first package.   now we divide 15.37/21.   in the first package, we have 0.73 dollars per ounce.now, let's look at the second package.   we start by multiplying 1.4*12=16.8.   there are 16.8 ounces in the package.   now, we divide 15.35/16.8=0.91.   so, in the second package, we have 0.91 dollars per ounce.the cost per ounce of the 10-pack is $0.73 and the cost per ounce of the 12-pack is $0.91.   the first package has the better price per ounce.the better explanation is the second one, because i prefer the lower price per ounce, i think that the 1st pack is the better buy.