Challenge Level

Notice that there is a total of $100\times100=10 000$ little squares in the grid.

The red line cuts the grid in half.

On each row, there is half of a shaded square on the 'wrong' side of the red line.

So it is $\frac12$ shaded plus $\frac{\frac12}{100}=\frac1{200}$ for the half square per line.

$\frac12 + \frac{1}{200} =\frac{101}{200}$

Add one more row so that the new shape is half shaded:

Grid on right: $100\times101=10 100$ squares, $10 100\div2=5050$ are shaded.

Grid on left: also $5050$ shaded, out of only $10 000$

$\therefore$ fraction shaded is $\frac{5050}{10000}=\frac{101}{200}$

Move shaded squares to make full rows (and empty rows):

$1$ square is moved into the row with $99$ shaded,

$2$ into the row with $98$ shaded

...

$49$ are moved into the row with $51$ shaded

$50$ are left in the row with $50$ shaded

Total $50\times100+50=5050$ shaded squares (including the row which already had $100$ shaded)

Fraction shaded: $\frac{5050}{10000}=\frac{101}{200}$

You can find more short problems, arranged by curriculum topic, in our short problems collection.