The mean of gregs journey times to work on 3days was 28minutes if it took him 27min and 25min on two of the days how long did it take him on the third day

In order to work out the mean, we add all the values together and then divide by how many values we had. In this case we had
(27 + 25 + x)/3 = 28

To work out what the third day (x) was, we must work backwards and do the opposite.
If finding the mean was adding then dividing, to find the value, we must multiply and then subtract.
Instead of diving by 3, we multiply by 3
28 x 3 = 84
Next we subtract the two values we already have, from 84
84 - 27 = 57
57 - 25 = 32 minutes

On the third day, Greg took 32 minutes to get to work.

Mean means average basically you do (total amount over time)/(total time)
in this case
(25+27+x)/(3)=28
solve for x
multiply both sides by 3 to clear fraction
25+27+x=84
add like terms
25+27=55
52+x=84
subtract 52 from both sides
x=32

he worked 32 mins