On an algebra test, the highest grade was 42 points higher than the lowest grade. The sum of the two

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On an algebra test, the highest grade was 42 points higher than the lowest grade. The sum of the two grades was 138. Find the lowest grade.

Let h be the highest grade and l be the lowest grade. Set up the given equations:
(1) h = l + 42
(2) h + l = 138

Substitute (1) into (2)
l + 42 + l = 138

Combine l terms
2l + 42 = 138

Enter that equation into our equation calculator to get
l = 48

Substitute l = 48 into (1)
h = 48 + 42
h = 90