Joshua
if x = cement ,y = sand z = aggregate then x/2 = y/5 = z/7 then each ratio is equal to (x+y+z)/(2+5+7) = 7000/14 = 500
then x/2 =500 y/5 = 500 z/7 = 500 .......cross multiply to find x,y,z.................1000,2500,3500.............. .......
It seems that my Basics of Ratio needs some extra skillS. Even if 7/14 = x/2, 14x = 14, x = 1 which meanS I need 1 tonne of cement, I am still confused of why I add the ratios together, then divide the needed tonnes (7 tonnes) into them!
look, you have an amount of something (in our case 7 tonnes) which you have to divide in 2 part : 5 part :7 parts, which would be a total of 14 parts of that something,
7/14 = 0,5 (of something - in our case is 0,5 tonnes, or 500kg of something)
now you sum them up proportionally as it is required: (2 parts x 0,5) : (5 parts x 0,5) : (7 parts x 0,5), gives you 1 tonne : 2,5 tonnes : 3,5 tonnes,
do you get now?
dokrbb
remember the simple rule that if we want to divide a quantity V in the ratio say a:b:c:d:e etc. then each component is got by dividing the ratio by the sum of the ratios and multiplying it by the quantity. In this case the quantity of component with the ratio b will be [ b/(a+b+c+d+e)] * V
In the given question similarly you can get the quantity of cement = (2/14)*7 Tonnes etc