Let s be the distance to Grandma’s and v the speed they went to get to Grandma’s.
So s=8v, because it took them 8 hours to get there.
Coming back they travelled s+20 miles. Since s=8v we can write this as 8v+20 miles.
Their speed was reduced by 10mph so that’s v-10. It took them 10 hours to return so:
8v+20=10(v-10); 8v+20=10v-100; 120=2v so v=60mph. This is their average speed going to Grandma’s.