Answer:
Explanation:
Let x be the score on 4th test.
We have been given that Gavin's mean score on 3 math tests is 87.
Since we know that average is the sum divided by the count.
Let us find sum of scores on 3 math tests by multiplying 87 by 3.
![\text{Scores on 3 tests}=3* 87](https://img.qammunity.org/2020/formulas/mathematics/high-school/2i9fnjzr04bqos5qu76mkqbs05cmy6do16.png)
![\text{Scores on 3 tests}=261](https://img.qammunity.org/2020/formulas/mathematics/high-school/qecge2v9xlcxrsb2yvdv9jawlqd2l091uo.png)
Since Gavin needs the mean score on his 4 tests to be 90. This means score on 3 tests plus x divided by 4 will be equal to 90.
Therefore, the equation
can be solved to find the score that Gavin needs on his test to bring his mean score up to 90.