Firstly, consider how many tickets she can purchase with $1590.

1590 ÷ 20 = 79 R10

She can purchase 79 tickets and have $10 left.

For every 10 tickets purchased, Ivy will get 1 additional ticket free.

Now for 30 tickets, she will get another 3 additional tickets free.

30 = 10 + 10 + 10

That's three sets of 10 tickets, which means:

For every 30 tickets Ivy purchases,
1 + 1 + 1 + 3 = 6

She will get 6 more tickets free.

79 - 30 = 49
49 - 30 = 19
19 - 10 = 9

These 79 tickets can be regrouped into two sets of 30 tickets, one set of 10 tickets and one set of 9 tickets.

From those two sets of 30 tickets, she will get two sets of 6 additional tickets free.
From that one set of 10 tickets, she will get 1 additional ticket free.
The last set of 9 tickets won't get her any additional free tickets.

Total tickets she can receive = 79 + 6 + 6 + 1
= 92

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