Question 747029
Try to keep all units as feet and pounds, or including cubic feet for volume.  Recognize for convenience that 18 inches is one and a half feet.  


Volume of the sandbox to fill is {{{7*7*(3/2) ft^3}}}.


Use the ratio of 50 pounds .... what you need to know is 1 bag per 0.70 ft^3.  You might also use it as 0.70 cubic feet per 1 bag.  


The needed expression:
{{{7*7*(3/2)*(1/0.70)}}}  and note that units are as {{{(feet*feet*feet)*(bag/feet^3)=bag}}}



That is 73.5 cubic feet, and 105 bags 
(?)