a)

Square tiles : 25%

The rest 100% - 25% = 75%

Since triangular tiles are 18 fewer than square tiles,

This means triangular tiles = 25% - 18

If we change 18 round tiles to triangular one triangular tiles becomes 25%

Then round tiles becomes 100% - 25% - 25% = 50%

Round tiles becomes 158 - 18 = 140

So this tells us 140 tiles is actually 50% of total.

Total is 100% = 140 x 2 = 280


b)

Increase by 25 % :

25% of 280
= 280 x 25/100 or 280 x ¼ or 280 ÷ 4
= 70

There is an increase of 70 square tiles

Initially 25% of tiles are square tiles. That's also 70 tiles.

So total number of square tiles now is 70 + 70 or 70 x 2 = 140

Total tiles now = 280 + 70 = 350

Percentage of total now that are square tiles

= 140/350 x 100%
= 2/5 x 100%
= 40%



Shorter method :

25% of tiles at first are square tiles. The increase of 25% are also square tiles.

100% + 25% = 125%

Total square tiles now : 25% + 25% = 50%

Percentage of total now that are square tiles

= 50/125 x 100%
= 2/5 x 100%
= 40%