A shuffle through the Gaming mailbag:

Q. Explain to me why this roulette system doesn’t work.

I bet \$10 on black and \$1 each on five different red numbers. That way, I have 23 numbers that win and only 15 numbers that lose. I win \$10 on each of the 18 black winners and \$35 on each of my reds. That’s \$255. I lose \$15 on each of the 15 losers, and that’s only \$225.

It looks to me like I should win \$30 more than I lose. You say combinations can’t beat the house edge, but it looks to me like they do.

A. On each of your winners, the other bet loses. On your 18 black winners, your \$10 bet is paid \$10, but you also lose \$5 on your red bets. That leaves your profit on each of those spins at \$5. Similarly, on your red winners, the black bets lose, so your profit on those spins is \$25, not \$35.

Let’s look at 38 spins of a double-zero wheel in which each of the 38 numbers comes up one. You risk \$15 per spin, so you have a total of \$570 in wagers.

On each of the black winners, you keep the \$10 bets plus your paid \$10. For 18 winners, that puts \$360 on your side of the table.

On each of the red winners, you keep \$1 and are paid \$35, and that leaves another \$180 with you.

All other bets are won by the house, including the red bets when you win on black and the black bets when you win on the red single numbers.

So at the end of the trial, the \$360 from your black winners and the \$180 from the red single numbers leave you with a total of \$540. Since your wagers totaled \$570, that leaves \$30 with the house.

Divide that \$30 by the \$570 risked, then multiply by 100 to convert to percent, and you get 5.26 percent. That’s the normal house edge on double-zero roulette.

Q. I notice you’ve answered a couple of questions about free slot apps like Jackpot Party Casino, Double Down Casino, Slotomania and others. Are you familiar with one called Mirrorball? It’s kind of fun, though a little limited. It doesn’t have anywhere near as many games as the others, but it has challenges and tournaments and you can win extra credits.

My problem is that it gives away so few credits compared to the others. I’m up enough levels in Slotomania that I get several hundred thousand credits every few hours. Mirrorball only gives a couple of thousand. Why so stingy?

A. Mirrorball’s more generous credit giveaways come through its themed challenges, which usually last about two weeks. It specifies 10 games as part of a challenge. When you play those games, at random times an overlay of two themed slot reels will spin to award you points toward mirrorballs. As you accumulate enough points to move up a level, you get mirrorballs and credits.

Each level is more difficult to attain and brings a bigger reward. At level 1, you’re awarded 8,000 credits and one mirrorball. At level 4, the top level, you receive 750,000 credits and four mirrorballs.

At challenge end, you may exchange the mirrorballs for credits plus charms that bring free spins on the game of your choice.

So it’s not so much a matter of being chintzy with the free credits as it is awarding the credits differently, as a reward for participating in the challenges, while awarding fewer credits just for checking in every few hours.