We need to compare 1/6 with 20%. Convert 1/6 to a percentage: 1/6*100=16.7%.
Or in terms of money 1/6*42=£7 while 20% of £42=20/100*42=£8.40.
Either way 20% off is a better offer from Soccer Supplies, where the cost is £42 less £8.40=£33.60 compared with £42 less £7=£35.