Lucky 15 means placing a bet on 4 events and choosing 15 different combinations as accumulator.
The 15 possible combinations in lucky 15 involves 4 singles, 6 doubles, 4 trebles, and 1 fourfold.

A single is a selection of just 1 event, double is a combination of 2 events, treble is a combination of 3 events and a four-fold is a combination of 4 events.
E.g:  You place a bet of lucky 15 of games A,B,C and D. Your stake is N50 but your total stake will be 50 X 15 = 750

Your combinations would be arranged thus:
4 singles
A X 50 = Winnings
B X 50 = Winnings
C X 50 = Winnings
D X 50 = Winnings

6 doubles
A*B X 50 = Winnings
A*C X 50 = Winnings
A*D X 50 = Winnings
B*C X 50 = Winnings
B*D X 50 = Winnings
C*D X 50 = Winnings

4 Trebles
A*B*C X 50 = Winnings
B*C*D X 50 = Winnings
A*C*D X 50 = Winnings
A*B*D X 50 = Winnings

1 four-fold
A*B*C*D X 50 = Winnings
You must get the prediction for at least one match correctly for you to get a return.

An example of a lucky 15 bet is displayed in the attached picture.