Series question (1 Viewer)

[nwa]

this question was in my exam for a) i got 920 everyone else got 880...and for b) i got 23 months everyone else got 24...

the attendance at classes in Gotham City has an average increase of 40 people per month. the orginal attendence was 200 people
a) how many people were attending classes after 1.5 years
b)How long until attendece of the classes reach 1120

 

[SoulSearcher]

a) Tn = a + ( n - 1 ) d

where a = 200 and d = 40 and n is measured in months

therefore, Tn = 200 + ( n - 1 ) * 40

1.5 years is equal to 18 months

Therefore T(18) = 200 + ( 18 - 1) * 40

= 200 + 680

= 880

b) let Tn equal 1120

therefore 1120 = 200 + ( n - 1 ) * 40

1120 = 200 - 40 + 40n

1120 = 160 + 40n

960 = 40n

therefore n = 24 months

 

[nwa]

but then if u put 24 back in the formula it wont work...
1120 -200 = 920
then 920 / 40 = 23

 

[PLooB]

but then if u put 24 back in the formula it wont work...
1120 -200 = 920
then 920 / 40 = 23

1120 - 200 = (n - 1) * 40

920 = (n - 1) * 40

23 = n -1

n = 24

Just remember 23 = n - 1 so n = 24