[問題]OG(11th)DS-Q110

[austi_co]

Whenever Martin has a restaurant bill with an amount between $10 and $99, he calculates the dollar amount of the tip as 2 times the tens digit of the amount of his bill. If the amount of Martin's most recent restaurant bill was between $10 and $99, was the tip calculated by Martin on this bill greater than 15 percent of the amount of the bill?
(1) The amount of the bill was between $15 and $50.
(2) The tip calculated by Martin was $8.

答案是B

但是小弟看了解釋也看不懂
請各位前輩賜教

[dibert8]

兩位數 = 10*a + b
tip = 2*a/(10*a + b) >? 15%

(1) a = 1~5, b = 0~9
if a = 1, b = 9 then tip = 2/19 < 15%
if a = 4, b = 0 then tip = 8/40 > 15%

(2) a = 4
if b = 0 then tip = 8/40 > 15%
if b = 9 then tip = 8/49 > 15%
8/40 < tip < 8/49 => tip > 15%

[austi_co]

真是清楚...
感謝dibert8前輩