The charge of ISD from Biratnagar to Tokyo is Rs 125 for the first 3 minutes and Rs 15 for every additional 10 seconds. If a man made a call from Biratnagar to his friend in Tokyo and paid Rs 305, how long did he make a call

We know he talked for over 3 minutes since the total is 305 which is > 125 (the cost of 3 minutes). The remaining amount he paid is 305-125= 180. He was charged 15 per 10 seconds which amounted to 180. 180/15=12 which means he was charged 12 times. Each time charged it was because he took 10 seconds so in total he took 12x10= 120 second. That’s 2 minutes. The two minutes he was charged 15s plus the first 3 minutes he was charged 125 is 5 minutes. We can plug in the numbers to make sure we are right. If he talked for 5 minutes then we know for the first three he was charged 125 and the additional two he was charged 15s (s representing 10 seconds). We can put the 125 aside for now to figure out the 15s. If he took two minutes then he took 120 seconds. Divide that by 10 because he’s getting charged per 10 seconds not per second so that’s 12. 15(12) is 180. 125 + 180 is 305

*** he took five minutes talking on the phone***

Yes you are correct