Answer:
91 squares
Step-by-step explanation:
It is adding on by 2, 4, 6, 8... So in the next figure (5), they would add on 8 squares. Now you have 13+8=21.
Then Figure 6. Add on 10. You have 31.
Figure 7. Add 12. You have 43.
8. Add 14. 57.
9. 16. 73.
Figure 10. Add 18. You get 91.
