Calculate the cost per square foot for each size tile.

1 square foot = 144 square inches.

Small tile = 4 x 4 = 16 square inches.

16 / 144 = 1/9 square foot.

Cost per square foot = 3.50 / 1/9 = $31.50 per square foot.

Large tile = 6 x 12 = 72 square inches.

72 / 144 = 1/2 square foot.

Cost per square foot = 4.50 / 1/2 = $9 per square foot.

The minimum area of the mosaic is 3 feet x 5 feet = 15 square feet.

The large tiles are cheaper per square foot.

Total tiles needed = 15 sq. ft. / 1/2 sq. ft = 30 tiles.

30 tiles x 4.50 each = $135