an=4n+1
Sum of first 30 terms
a1+a2+.................a30
Sn=(a1+an)*n/2
a1-----------first
terms
an=---------last
term-----a30
n= number of
terms-----30
calculation of a1
a1=4n+1=4*1+1=5
calculation of an
a30=4*30+1=121
S30=(a1+a30)*30/2
S30=(5+121)*30/2=1890
<span>The sum of first
30 terms is 1890</span>
Holly has 20 balls, and there are 100 balls in total.
So there are 100-20 = 80 balls left.
Alex had 2 * the amount Holly had, which is 20.
So Alex has 20 * 2 = 40 balls.
Together, they have 60 balls.
Which means 100-60 = 40. So Ben has 40 balls.
Check:
Holly: 20
Alex: 40
Ben: 40
Add them: 20 + 40 + 40 = 100
It opens downwards so it looks like this “n”. That is because a in the formula is a negative number in this situation