What percentage of the map would have unhealthy levels of arsenic (140 total squares)?

35

Explanation:

so you said 140 in total and it's a square

it's basically finding the perimeter of the square

so 35 is each side of the square that I am imagining in my head

so if you add 35...4 times you will get 140

for example:35

35

35

+ 35

140

or 35×4=140

can you make it more obvious. i can't see it properly