.. which reminds me of my O level paper.
A square flag has a blue field with two white diagonal bars that form a cross. Each bar extends across the entire diagonal, from corner to corner. The cross occupies half of the flag's area. The flag measures 60 inches on each side. To the nearest 0.01, what is the width of the bars?
21 and a bit? Any closer than that and the crayons get it.
The flag is 60 x 60 = 3,600 sq inches.
That mean it got 4 triangles each 900 sq inches. Area of a triangle is 1/2 base times height ie 60/2 x 30 = 900. You does see the height is the square root of the area in this triangle.
If the crosses have half the area, then each triangle is 450 sq inches. It a similar triangle to the big one, so for the height you want the square root of 450 and double the answer for the base of the triangle. Square root of 450 is 21.21 (it actually 449.8641). So each triangle have base of 42.42 inches, with 8.79 inches each side of the base.
This 8.79 inches itself one of 2 equal sides of a 90 degree angle triangle at each corner of the flag, so to get the hypotenuse (which will be the width of the saltire) you want the square root of (2 x (8.79 x 8.79)) = square root of 154.53 = 12.43 inches.
So the answer is 12.43 inches, wergh.
That hard, I got it wrong first time.
Helio - 'the crayon gets it' 
Midge, you deserve an extra helping of Hills for working that one out! 