Ezra is training for a track race. he starts by sprinting 100 yards. he gradually increases his distance, adding 5 yards a day for 21

This is the concept of sequences and series, the distance in which Ezra would be running after 21 days will be given by:
an=a+d(n-1)
where;
a=first term=100 yards
d=common difference=5
n=number of terms=21
thus the distance after 21 days will be:
a21=100+(21-1)5
=100+20*5
=100+100
=200 yards