If Mohan made STD calls (within 100 kms) for 30 min and spent 100 min on local calls with 30% on landline, 40% in GSM and 30% on Airtel, and Rohan spent 18 min on STD within 150 Kms and spent 120 mins on local calls with 30%, 40% and 30% on GSM, landline and Airtel respectively, then who spent more?

For Mohan, 

There are 2 cases for STD calls:

1) Call rate of 1.5 which will give 30*1.5= Rs. 45

2) Call rate of 2, which will give 30*2= Rs. 60

For Local calls,

30% of 100min in landline= 30 min of landline will cost 30*2= Rs. 60

40% of 100 min in GSM = 40 min of GSM will cost 40*1= Rs. 40

30% of 100 min in Airtel= 30 min of Airtel costs 30*1= Rs. 30

Total money spent by Mohan for local calls= Rs. 130.

In total he spends either Rs. 130+ 45= 175 or Rs. 130+60= Rs. 190.

For Rohan, 

The two cases for STD call rates will cost him:

1) 18*1.5= Rs. 27 with call rate of Rs.1.5/min

2) 18*2= Rs. 36 with call rate of Rs. 2/min

For Local calls,

30% of 120 min= 36 min with GSM will cost him Rs. 36.

40% of 120 min= 48 min with landline will cost him Rs. 48*2= Rs. 96

30% of 120 min= 36 min with Airtel will cost him Rs. 36.

Total money spent on local calls= Rs. (36+ 96+36)= 168.

Including the STD calls, he spends either Rs. 195 or Rs. 201.

Clearly, even the case where the maximum money spent by Mohan is less compared to the case where the minimum amount spent by Rohan is considered. Therefore, Rohan spends more of the two.