Ex 5.2, 20 - Chapter 5 Class 10 Arithmetic Progressions

Transcript

Ex 5.2 ,20 Ramkali saved Rs 5 in the first week of a year and then increased her weekly saving by Rs 1.75. If in the nth week, her weekly savings become Rs 20.75, find n. Saving made first week = 5 Saving made in second week = 5 + 1.75 = 6.75 Saving made in third week = 6. 75 + 1.75 = 8.50 So, the series is 5, 6.75, 8.50 Since difference is same, it is an AP Common difference = d = 1.5 Given, in nth week, her savings become Rs 20.75 So, an = 20.75 Also, a = 5 , d = 1.75 We need to find n Putting these in formula an = a + (n 1) d 20.75 = 5 + (n 1) 1.75 20.75 5 = (n 1) 1.75 15.75 = (n 1) 1.75 15.75/1.75 = n 1 1575/175 = n 1 9 = n 1 n 1 = 9 n = 9 + 1 n = 10 Hence, in 10th week, her weekly savings become Rs 20.75