Sign up to betting site PaddyPower and deposit £10. You will then get £16.50 cashback from TopCashback. Currently, Paddypower will refund your stake if it loses, so you should even get get your initial £20 deposit back, or win even more!
You could also look into ‘Matched betting’, although slightly complicated to guarantee yourself a profit of £15 on the bet plus the £16.50 Topcashback. Do remember that this one is about free cashback though, and not silly gambles!