Let x be the number of ear corns sold and let y be the cost.
Step 1.
We know that the constant of proportionality is given by:

Al's stand.
In this case we know that when they sell 6 ears the cost is 1.50, then we have:

Hence, the constant of proportionality for Al's stand is 0.25
Barbara's stand.
For this stand we have that:

Hence, the constant of proportionality for Barbara's stand is 0.24
Step 2.
Once we have the constant of proportionality we can write the equation for each stand.
Al's stand:

Barbara's stand:

Step 3.
Al's stand table.
If they sell 14 ears, then:

If they sell 21 ears, then:

If the total cost is $50 then:

Barbara's stand
If they sell 14 ears:

Is they sell 21 ears:

If the total cost is $49.92 then:

Step 4
The constant of porportionality for this problem gives us the price per ear of corn.
Step 5
Barbara's stand has the better value, this comes from the fact that the constant of proportionality is smaller for this stand; which means that the cost for each ear of corn is less.