A certain library assesses fines for overdue books as follow

You are correct and here too increase by means 0.1 +0.3 or 0.4, but doubled gives 0.1*2 or 0.2.
As we are looking for least amount, we take 0.2 and continue the same for next days too.

Hi kadamhari825,

The prompt tells us that one of two possible actions occurs each day:

"for each additional day....., the total fine is EITHER increased by 30 cents OR doubled, whichever results in the LESSER AMOUNT." This means that on certain days, we will ADD to the total and on other days, we will MULTIPLY the total by 2.... whichever results in the smaller total.

Since we're asked to calculate a book that is 4 days overdue, we just have to consider each possible outcome on each day, then choose the one that leads to the "lesser amount"

Day 1: 10 cents

Day 2: 10 + 30 = 40 cents; 10(2) = 20 cents
20 cents is LOWER, so the fine is NOW 20 cents

Day 3: 20 + 30 = 50 cents; 20(2) = 40 cents
40 cents is LOWER, so the fine is NOW 40 cents

Day 4: 40 + 30 = 70 cents; 40(2) = 80 cents
70 cents is LOWER, so the final fine is 70 cents.

GMAT assassins aren't born, they're made,
Rich

The catch here is that the fines are cumulative.

Fine on the first day is $0.1

Fine on the second day is 0.1*2 = $0.2 (as doubling gives us a lower value in comparison to fine increased by $0.30)

Similarly, fine on third day is 0.2*2 = $0.4 (as doubling gives us a lower value)

On the fourth day fine is 0.4 + 0.3 = $0.7 (as doubling gives higher value and thus we add $0.3)

Thus the fine overdue on fourth day is $0.4.
(option b)

D.S
GMAT SME

Total day one: 0.1

Total day two: [0.1] + 0.1

Total day three: [0.1 + 0.1] + 0.2

Total day four: [0.1 + 0.1 + 0.2] + 0.3 = 0.7