Answer:
 of mulch and 
 of gravel were sold.
Explanation:
Mass of mulch sold = x pounds
Mass of gravel sold = y pounds
Total mass of mulch and gravel sold = 173 pounds
x + y = 173 ...[1]
Supplier sells 
 pounds of mulch for every 
 pounds of gravel.
That is for 
 pounds of mulch for every 
 pounds of gravel.
Then gravel sold for 1 pound mulch = 

Then for x pounds of mulch the gravel sold will be:

Mass of gravel sold = 

Putting value of y in [1] :

x =

y = 

 of mulch and 
 of gravel were sold.