See if we have N amount of the sample. Then after 45 minutes N/2 of the sample decays. Now after another 45 minutes, the N/4 of the sample decays.
Thus the total decay in 90 minutes is N/4+N/2=3N/4. Thus 3/4 part of the sample decays in 90 minutes.
I hope this answer helps. All the very best for your future endeavors!
Question : If $[4-\frac{5}{1+\frac{1}{3+\frac{1}{2+\frac{1}{4}}}}]$ part of a journey takes 10 minutes, then to complete $\frac{3}{5}$ th of a journey, it will take:
Option 1: 40 minutes
Option 2: 45 minutes
Option 3: 48 minutes
Option 4: 50 minutes
Question : Two pipes, A and B, can fill a tank in 36 minutes and 45 minutes, respectively. Another pipe, C, can empty the tank in 30 minutes. Initially, pipes A and B are opened. After 7 minutes, pipe C is also opened. The tank is then filled up in:
Option 1: 39 minutes
Option 2: 46 minutes
Option 3: 40 minutes
Option 4: 45 minutes
Question : Walking at the rate of 4 km/hr, a man covers a certain distance in 3 hours 45 minutes. If he covers the same distance on a cycle, cycling at the rate of 16.5 km/hr, the time taken by him is:
Option 1: 55.45 minutes
Option 2: 54.55 minutes
Option 3: 55.44 minutes
Option 4: 45.55 minutes
Regular exam updates, QnA, Predictors, College Applications & E-books now on your Mobile