Final answer:
To make 1 whole pie, Joe would need 1 1/4 teaspoons of salt since he uses 1/8 teaspoon for each 1/10 of the pie.
Step-by-step explanation:
Joe used 1/8 of a teaspoon of salt to make 1/10 of a pie. To find how many teaspoons of salt it takes to make 1 whole pie, you need to multiply the amount of salt used for 1/10 of the pie by 10, because there are 10-tenths in a whole. Therefore, if 1/8 of a teaspoon is needed for 1/10 of a pie, you need 10 times that amount for the whole pie, which is 10 × (1/8) = 1 1/4 teaspoon of salt for one whole pie.