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Take "n" as the limit for "Rs. 300".
Case1(When one is using):
per call rate after crossing the limit(i.e n)=(1550-300)/(1400-n)

Case2(When only both are using)
per call rate after crossing the limit(i.e n)=((425+925)-(300*2))/(1400-2n)

so 1250/(1400-n)=750/(1400-2n)
. n=400

x is number of calls made by first guy, thus 1400-x by second guy
T is threshold for calls being charged 350
R is rate per call beyond threshold

(x-T)*R          = 125            = 425-300
(1400-x-T)*R     = 625       = 925-300
(1400-T)*R        = 1250       = 1550-300

3 equatiosn 3 unknowns easy

Take "n" as the limit for "Rs. 300".

u1 = number of calls by person 1 = n + k1.
u2 = number of calls by person 2 = n + k2.
R = Rate in Rs/call

p1 = price paid by person 1 = 425 = 300 + k1 * R => k1R = 125
p2 = price paid by person 2 = 925 = 300 + k2 * R => k2R = 625


Result1: k2 = 5 k1 and 6k1 * R = 750

Total calls = 1400 ==> n + k1 + n + k2 = 2n + 6k1 = 1400
Result2: n + 3k1 = 700

u = calls by one person = 2n + 6k1 = 1400
p = price by one person = 300 + (n + 6k1) * R = 1550 ==> n * R = 1550 - 750 - 300 = 500
Result3: n * R = 500

Result1 & Result 3 => Result 4: k1 / n = 125 / 500

Result2 & Result4 => (1 + 125 / 500 * 3) * n = 700 ==> n = 500

N is the number of telephone calls allowed for a lumpsum charge of Rs.300.
R is the rate beyond that.
You would get equations like the following.
2N+(425-300)/R+(925-300)/R=1400
(1400-N)R+300=1550
You can easily get NR and R from the above equations and you would get N eventually.
N = 400

n is the number of telephone calls allowed for a lumpsum charge of rs.300.
r is the rate beyond that.
You would get equations like the following.
2n+125/r+625/r=1400
2nr+ 125+625 = 1400r
2nr + 750-1400r=0 --1
(1400-n)r=1250
1400r -nr -1250 =0 --2
now multiply 2 by 2 and add both 1 nd 2
1400r - 1750 =0
r = 1750/1400 = 1.25
so n = 400

Let x be the no of cal allowed for 300 and r be the rate after that

so we get 2 equations

1. x + (425 - 300)/r + x + (925 - 300)/r = 1400
2. x + (1550 - 300)/r = 1400

solving for x and r , we get x =400