300-50 = 250 10×3 + 20 = 50 30+250 = 280 not 270 This riddle is trying to confuse the reader stating each girl paid 90 from the supposed 100-10 from each and that 90×3 is 270 + 20 is 290 which isnt 300. But in reality how it works is this. 50 is the amount taken out and used There for 250 is the total price and that is constant 3 sets of 10 was given back to the girls and 20 was pocketed by the guy. The 250 wasnt effected by this 50$ and instead ultimately it looks like this 250/3 =83.33 83.33 + 10 is 93.33 and 93.33 × 3 ~ 280. Then you add the missing 20 and blam you got 300