Question

on the 1st of January a student puts N10 in a box. on the 2nd she puts N20 in the box. putting the same number of 10 naira notes as the day of the month. how much money will be in the box if she keeps doing this for the first 10 days of January and the whole of January​

Answer 1

N550

N4960

Explanation:

10 days

First day: N10

Second day: N20

Third day: N30

...

Tenth day: N100

Sum of sequence: 10 + 20 + 30 + 40 + ... + 80 + 90 + 100

sum = (n/2)[2a+(n−1)d]

a = 10; n = 10; d = 10

sum = (10/2) × [2(10) + (10 - 1)(10)]

sum = 5[20 + 9(10)]

sum = 5[110]

sum = 550

Answer: N550

31 days

First day: N10

Second day: N20

Third day: N30

...

Thirty-first day: N100

Sum of sequence: 10 + 20 + 30 + 40 + ... + 290 + 300 + 310

sum = (n/2)[2a+(n−1)d]

a = 10; n = 31; d = 10

sum = (31/2) × [2(10) + (31 - 1)(10)]

sum = 15.5[20 + 30(10)]

sum = 15.5[320]

sum = 4960

Answer: N4960