The math problem that would give you your maximum heart rate if you are 13 years old is 220 - 13.
Therefore, your maximum heart rate would be 207 beats per minute.
The lower range of your target heart rate zone would be 70% of your maximum heart rate:
0.7 x 207 = 145 beats per minute.
The upper limit of your target heart rate zone would be 85% of your maximum heart rate:
0.85 x 207 = 176 beats per minute.