Which price is cheaper? 20% off the original price of $29.99 or 10% off the original price of $26 for a book?

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