First you purchase 7 BTC on Low price. When Price increase then you will sell your BTC and wait for Price down when price down purchase More BTC then you will make 17BTC out of 7 BTC.
You could simply say he could do it by trading, you didn't even need to do that as many people have already said that.
I think trading requires a long time to make it all, but I agree it's better to play trading a fairly safe compared with gambling.
indeed both are equally at risk, at least trading will not lose all in an instant.