Let the first grade be x then the next will be x + 42
Given that the sum of the two grades is 138 then
x + x + 42 = 138
2x + 42 = 138
Divide through by 2
x + 21 = 69
x = 69 - 21
x = 48
The second number
= 48 + 42
= 90
Hence the numbers are 48 and 90
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