How to turn this word problem into an equation

Erik has 8 sacks of flour. Some of his sacks weigh 3 pounds and the others weigh 7 pounds. The total weight of his flour is 36 pounds. How many 7-pound sacks does he have?

I figured out the answer, but I want to know how it'll look like as an equation.


Thanks! :)

3x + 7y = 36
x + y = 8

3x + 7y = 36
- 3x - 3y = - 24
4y = 12
y = 3
x = 5

5 of the 3 lb bags
3 of the 7 lb bags

Use algebra. Let x be the number of 7-pound sacks he has.
(3)(8-x) + (7)(x) = 36
24 - 3x + 7x = 36
24 + 4x = 36
4x = 12
x = 3

Let n be the number of 7 pounders .. so 8 - n is the number of 3 pounders
thus 7n + (8-n) 3 = 36 <------ Your desired answer ...

which gives n = 3
so 3 of the 7 pounders and 5 of the 3 pounders