To find the distance Mary will walk diagonally across the rectangular lot, we can use the Pythagorean theorem. The formula is:
\[ c = \sqrt{a^2 + b^2} \]
where \(c\) is the length of the diagonal, \(a\) is the length of the rectangle, and \(b\) is the width of the rectangle.
In this case, the lot is 120 feet long and 50 feet wide. So, we can substitute \(a = 120\) feet and \(b = 50\) feet into the formula:
\[ c = \sqrt{120^2 + 50^2} \]
Calculating \(120^2\) and \(50^2\):
\[ 120^2 = 14400 \] \[ 50^2 = 2500 \]
Now add these two results together:
\[ 14400 + 2500 = 16900 \]
Now take the square root of \(16900\):
\[ c = \sqrt{16900} = 130 \]
Therefore, the distance Mary will walk diagonally across the lot is 130 feet.