m = charge per unit of call , b = fixed charge
T = m N + b
806 = m * 420 + b
410 = m * 200 + b
subtracting equations ... 396 = m * 220 ... m = 9/5 = 1.8
substitute back to find b
The charge for telephone calls T is partly constant and partly varies with the number N of units of call. The bill for 420 units of call is 806 naira, while the bill for 200 units is 410 naira.Obtain the charge per unit of call, find a formula for T
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