Saving in January

On the 1st of January a students puts $10 in a box. On the 2nd she puts $20 in the box, and so on, putting the same number of 10-dollers notes as the day of the month. How much money will be in the box if she keeps doing this for

a the first 10 days of January
b the whole of January?

Correct answer:

a =  550
b =  4960

Step-by-step explanation:

a1=10 d=10 n1=10 a10=a1+(n11) d=10+(101) 10=100  a=a1+a102 n1=10+1002 10=550
n2=31 a31=a1+(n21) d=10+(311) 10=310  b=a1+a312 n2=10+3102 31=4960

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