Quant interview question I got yesterday

[fierce venture]

you use all coupons on the first coin until it flips 10, then the remaining coupons go on 2nd coin etc. that's the strategy

if you used 1 coupon, there's 5 ways to do that, each with 2^−6 probability

if you used 2 there's 6c2 ways each with 2^−7

in each case up to 4 used you win maximum idk what happens if you used all 5