And increase the chance of it failing on that particular day!
Taking an average of 1.15 would require 15 consecutive winners. Again, assuming the true odds are 10% away from the price, the probability of success is still around 10% i.e. 10/1 odds for a 15/2 payout.
Whichever way you cut it, it doesn't make financial sense to bet like this. Even within the arbitrary confines of seeking to tur £20 into £150 in a month, your best chances of doing that are to put it all on a 15/2 shot to win, where the true odds are as close as possible to the price (or, very rarely, in your favour). By combining bets, you are leveraging the bookie's spread between the price and the true odds, over and over again. Whilst it has an instinctive attraction as a money-making scheme, the maths show it's very, very poor. (Though it might be fun.)