In order to answer this question you need to find out how much cement, sand and gravel is needed to make 38.5g of concrete mix. To do this you must find out how many parts the concrete mix ratio is made up (4 + 2 + 1 = 7) Then divide the total concrete mix (38.5kg) by the number of parts; 38.5/7 = 5.5. This tells you that each of the 7 parts of the ratio = 5.5kg. As the ratio tells you sand is 2 parts you multiply this by 5.5 (2 x 5.5kg = 11kg).
Finding the greatest common factor ( or G C F ): G C F is the largest number that divides evenly into all the numbers. 270 : 90 = 3 360 : 90 = 4 G C F ( 270, 360 ) = 90
