Let 'x' be the number of tickets Sally bought.

Sally has $10.

The equation will look like this:

1 + 3 * x = 10

At the entrance Sally paid the entry fee of $1 and is left with 10 - 1 = 9 dollars

After the first ride she has spent $3 and is left with 9 - 3 = 6 dollars

After the second ride she spent another $3 and now has 6 - 3 = 3 dollars

And finally she spends the last $3 on the her last, third, ride.

Thus we conclude that Sally has bought 3 tickets.

We get the same result if we solve the equation:

1 + 3x = 10

3x = 10 -1

x = 9 / 3

x = 3 tickets.