Cost of using the smaller tiles
Area = 210 * 90 = 18,900
Total number of tiles required = 18,900 / 100 = 189
That is 5/7 * 189 = 135 white tiles and 54 coloured tlles
Cost = 135*3.99 + 54*4.99 = £808.11
Larger Tiles:-
Total numbers of tiles = 18,900 / 225 = 84
= 5/7 * 84 = 60 white tiles and 24 coloured tiles
Cost = 60*8.99 + 24 * 9.99 = £779.16
So using the 15 cm square tiles would be cheaper.