Question

The cost of drilling tubewell is a 2.50 per foot for the first 100 feet and an additional Re.0.25 for every subsequent foot .find the cost of the last foot and the total cost of 220 feet deep tubewell.​

Answer 1

Step-by-step explanation:

cost for the first 100 feet = rs 2.50×100 = rs 250 For the next 120 feet, cost are in A.P : 2.75, 3.00, 3.25,.... here, a=2.75 , d=0.25, n=120 Sn= n/2[2a+(n-1)d] = 120/2[2×2.75+(120-1)0.25] = 60[5.50+(119)0.25] = 60(5.50+29.75) = 60(35.25) = 2115 So, total cost of 220 feet = rs250+rs2115 = rs2365 Cost of last foot: By using the formula, Tn = a+(n-1)d T120 = 2.75+(120-1)0.25 = 2.75+29.75 = 32.50 So, cost of last foot = rs32.50