Pleaaase help D:

Davids scores on the first three of four 100-point math tests were 86, 92, and 89. What score does he need on his fourth math test to ensure an average score of at least 90?

A. x 89

B. x 93

C. x 78

D. x 90

Question

Pleaaase help D: 

Davids scores on the first three of four 100-point math tests were 86, 92, and 89. What score does he need on his fourth math test to ensure an average score of at least 90?

A. x > 89 

B. x > 93

C. x > 78

D. x > 90

anonymous · Answer

D.
Find the average of the already given scores. $$\frac{86+92+89}{3} = 89$$
David's scores are equivalent to three 89 scores.
The wanted score of 90 is equivalent to four 90 scores. x is the score of the test we want.
$$3(89)+x=4(90)$$ which  is$$267+x=360; ~ \therefore~~ x=360-267 \therefore~x=93$$
93 is greater than 90 but not greater than 93.

anonymous · Answer

(Finding 89 is not necessary; we could have just added 86+92+89)

anonymous · Answer

(86 + 92 + 89 + x) / 4 >= 90
267 + x  >= 360
=> x >= 93

anonymous · Answer

B. x > 93