As henry has mentioned this is a problem that is similar to the one discussed in class.
First bar purchased = 1 coupon
1 = number of extra bars for a different coupon 1 is a geometric Random variable
= # of extra bars after 2nd coupon to get 3rd coupon
X3 is geom
X4 is geom
Xn is geom
Avg. # of coupons E[# needed]=
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