[MATH]5[/MATH] sand : [MATH]2[/MATH] cement
means everytime you use 5 sands, you must use 2 cements
if you will use [MATH]7[/MATH] bags
then the ratio is
[MATH]5:2[/MATH]
if you will use [MATH]14[/MATH] bags
then, you multiply both sides by two
[MATH]2 \cdot 5 : 2 \cdot 2[/MATH]
How to know which number to use for multiplication?
the formula is simpe = [MATH]\frac{number \ of \ bags}{total \ ratios}[/MATH]
so above, we used [MATH]2[/MATH] because [MATH]\frac{14}{5 + 2} = \frac{14}{7} = 2[/MATH]
Now if they told you that you will use 13 bags, what are the ratios?
For the 2nd question
Can you show us what did you try?
