Jeremy mows lawns during the summer in his neighborhood. jeremy has made $800 mowing 35 lawns. he charged either $20 or $25 a lawn. for the 35 lawns mowed, how many times did jeremy charge $20?

Given that Jeremy mows two types of lawns during the summer in his neighborhood.

Let number of lawns that are charged at $20 = x

then total earning from x such lawns = 20x

Let number of lawns that are charged at $25 = y

then total earning from y such lawns = 25y

Then his total earning = 20x+25y

Given that he earned $800 so we get equation:

20x+25y=800...(i)

Since total number of lawns are 35 so we get:

x+y=35

or x=35-y...(ii)

Plug (ii) into (i)

20(35-y)+25y=800

700-20y+25y=800

700+5y=800

5y=800-700

5y=100

y=100/5

y=20

Now plug value of y=20 into (ii)

x=35-20=15

Hence Jeremy charged 15 lawns at $20.