We know that it receives $55 per race entry, but it must spend $15 per race entry to cover the cost of the race.

So, it really receives $40 per race entry.
We also know that it receives $10,000 in donations.
If the charity needs to raise at least $55,000, to find the number of race entries that it needs we can use the next inequality

Where, x represents the number of race
Finally, we must solve the inequality

So, the charity needs 1125 race entries to raise at least $55000