To find the average miles per gallon, you divide the distance traveled by the gallons of gas consumed.
In this case the truck went 418 miles on 21 gallons. So we get the division sentence `418-:21=19.9047619047619` mpg.
The problem then tells us to round to the nearest tenth. The tenth is the place-value just to the right of the decimal point (.) We check the next digit to the right: 0 in this case. Since 0<5 (Five or more, up the score) we round down i.e. keep the tenths digit the same and reduce all digits to the right to zero.
This gives us a final answer of 19.9 mpg.