If I spend $11 on 880 robux twice a month and $9.99 on 1000 once every month, How much would I spend for 17000?
880 robux cost $11, so 17000 robux would cost:
17000/880 = 19.31818181818182
Rounding up to the nearest whole number (since you can't buy a fraction of a robux), you would need to buy 20 sets of 880 robux.
20 sets of 880 robux would cost:
20 * $11 = $220
Plus the $9.99 for the 1000 robux once a month:
$220 + $9.99 = $229.99
So you would spend $229.99 for 17000 robux.