20% off means that now it costs 80% of original price
80% = 0.8
costs 80% of (means here times (*))original price
29.99*0.8≈23.99
10% off means that now it costs 90% of original price
90%=0.9
costs 90% of (means here times (*))original price
26*0.9=23.40
23.99>23.40,
so is
10% off the original price of $26 is cheaper