Mr. Abbot mowed 1/4 of his lawn.
His son mowed 3/7 of the lawn.
The first thing to do is get a common denominator, in this case a good one to use is 28. it has to be a number divisible by 4 and 7.
1/4=7/28
3/7=12/28
so now we can see that the son mowed more. now add these two numbers together to get how much they mowed in total
(7+12)/28=19/28 of the lawn is mowed.
that means 9/28 of the lawn still has to be mowed.
