A student scores 80, 90, and 70 in three subjects. What score does he need in the fourth subject to have an average of 85?

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Q: A student scores 80, 90, and 70 in three subjects. What score does he need in the fourth subject to have an average of 85?

Solution: Let the score in the fourth subject be x. The average is (80 + 90 + 70 + x) / 4 = 85. Thus, 240 + x = 340, so x = 100.

Steps: 8

Step 1: Add the scores of the first three subjects: 80 + 90 + 70.
Step 2: Calculate the total from Step 1: 80 + 90 + 70 = 240.
Step 3: Let the score in the fourth subject be x.
Step 4: To find the average of four subjects, use the formula: (Total of all scores) / (Number of subjects).
Step 5: Set up the equation for the average: (240 + x) / 4 = 85.
Step 6: Multiply both sides of the equation by 4 to eliminate the fraction: 240 + x = 340.
Step 7: Solve for x by subtracting 240 from both sides: x = 340 - 240.
Step 8: Calculate the value of x: x = 100.