If you mean that the total number of questions is 70, then:
1. You need to figure out what 90% is of the total, 70.
2. Do this by multiplying 70 by 90% --> 70*(.9)
3. 90% of 70 is 63. The total number correct is 63.
You can check your work by figuring out what percentage 63 is of 70. 63 divided by 70 (63/70) is .9, or 90%.