### GMAT Practice Question Source:

### Difficulty: Sub-600 Level

### GMAT Study Guide Module:

### GMAT Timing Tips:

**Weighted average mapping strategy**: Questions like this, where 1 more item is being added to a group, can be solved using the weighted average mapping strategy. One weight will be the number in the original group (4 in this question), and the other weight will be 1, the size of the “group” of one being added; these are shown above the line in the diagram below. We know the average of the original group (78) and the final weighted average (80), so we can fill these in below the line in the diagram. We need to solve for the “average” of the group of 1 (which will be the score on the 5th test) by making the ratio of the weights equal to the ratio of the distances. The ratio of the weights is 4:1, and the smaller distance is 2, so the larger distance must be 8. Thus, the score of the 5th test is 80+8 = 88.

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