simple word problem

ankitasingh wrote:A man earns Rs 20 on the first day and spends Rs 15 on the next day. He again earns Rs 20 on the third day and spends Rs 15 on the fourth day. If he continues to save like this, how soon will he have Rs 60 in hand?
(a) On 17th day
(b) On 27th day
(c) On 24th day
(d) On 30th day

Please provide the expalnation.

A: He earns $20 at the beginning of one $5 earning block. He get $5 in two consecutive day. SO he needs to save $40 and the next day he will get $20 reaching to $60 mark. To earn $40 $40/$5 = 8 two day blocks are needed.

So a total of 8*2+1 = 17 days.

if he needs Rs 60

rate of earnings Rs. 5 , 60/5 = 12 i.e he needs twelve 5's

he earns Rs. 5 once in two days, for twelve 5's in (12*2) 24 days

Ans (C)

whats the IMO?

Good question. But GMAT questions have five answer choices. And, as a North American outfit, GMAC will (for better or worse) use dollars instead of rupees. Maihuna's solution is correct. Every two days there is a net increase of 5 rupees. So, after 8 of these two day-cycles (after 16 days), he will have 40 rupees. On the 17th day, he will get another 20 rupees, and will have 60 rupees in hand.

If you were aware of the pitfall in this problem, you could have simply divided 60 by 5, arriving at 12 two-day cycles or 24 days; and you would know that the answer had to be less than 24; there is only one choice less than 24.