Let's begin by listing out the information given to us:
One square grid = 5 feet
The side of the square measures 10 grids = 5 * 10 = 50 feet
Squares have all sides congruent. This means that both the length & width of the square equals 50 feet
To obtain the length of the diagonal of the square, we will calculate using Pythagoras Theorem as shown below:
c² = a² + b²
a = 50 ft, b = 50 ft, c = ?
c² = 50² + 50² = 2500 + 2500 = 5000
c² = 5000 ⇒ c = 70.7
c = 70.7 ft
The length of the diagonal according to the plan is not 60 feet. Hence, it is wrong