# Find the ratio of sum of 24th term.

The ratio of sum of $n$ terms of two Arithmetic Progressions is

$$r_n= \frac{ 3n - 3}{5n + 21}$$

I'm asked to find the ratio of sum of $24$th term.

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What have you tried? –  Antonio Vargas Feb 24 '12 at 6:16
I begin with keeping n as 24. –  vikiiii Feb 24 '12 at 6:22
Is it $3n-(3/(5n))+21$, or is it $(3n-3)/(5n+21)$? anna, only you know for sure - please edit if what's there now is wrong. –  Gerry Myerson Feb 24 '12 at 6:28

You have one progression that is $a, a+d, a+2d, \ldots$ and another that is $b, b+e, b+2e, \ldots$ The problem promises that $\frac ba=0$ (assuming that the count starts with 1). BTW please put parentheses in your expression. I am sure you mean $\frac {3n-3}{5n+21}$ but you wrote $3n-\frac 3{5n}+21$