2. Let x be the lowest grade on the next quiz.
Since the average grade should be at least 18, the sum of all grades should be greater than or equal to (18 * 5), which is 90.
So, the inequality can be written as:
19 + 17 + 20 + 15 + x ≥ 90
3. Let L be the number of hours Luis worked.
Since Stacey worked 4 more hours than Luis, let S = L + 4 be the number of hours Stacey worked.
Together, they worked more than 28 hours, so the sum of their hours should be greater than 28.
The inequality can be written as:
L + (L + 4) > 28
4. Let Y be the amount the younger son saves.
Since the older son agrees to save twice as much as the younger one, the amount saved by the older son can be written as O = 2Y.
Together, they must save more than $45, so the sum of their savings should be greater than 45.
The inequality can be written as:
Y + O > 45
Y + 2Y > 45
3Y > 45
Note: It seems there is a mistake in your example solution for problem 4. Y should be the determinant for O, not the other way around. If Y saves 25, O should save 2Y, which is 50.