How do I find the mean in this problem

After 3 tests Jason has a mean score of 80. What will she need to get on the 4th test to bring his mean up to 85?

Thank you for your help!

Total marks so far must be 3 times 80 = 240 marks. You want to know what number, when added to this and the total divided by 4, will give 85.

(240+x)/4 = 85 Multiply both sides by 4

240 + x = 340 Subtract 240 from both sides

Jason needs to get 100 on the final test to lift his average to 85

The current mean is 80, the future mean is 85. since jason did three tests and the average is 80, 3*80 = 240, in order to make the mean go up to 85 with 4 tests. 4*85=340 the 4th test must be 340-240 or 100 to make the mean of 85.

mean = total / number of inputs

mean now is : 80 = tot / 3
so, now total is 240

mean later is to be 85 so 85 = total / 4
total later need to be 340

so you need 100 on the 4th grade to get from 240 total to 340 total and reach 85 ave

Total points so far = 3*80 = 240
Total needed = 4*85 = 340
Points needed = 100