pretest question #2

"The mean mark on 4 tests is 78%. What mark is needed on the next test to increase the mean to 80%?"

<===================================================>

78, 78, 78, 78 is possible for his test marks. so 78+78+78+78=312. And 312/2=4=78.

Knowing that 80x5 (the amount of datas we need) = 400, and 400-312(the previous sum of all data) =88, we need 88 as his mark. (78+78+78+78+88=400/5=80).

So 88% is needed on the next test to increase the mean to 80%.

-michael