We know that his mother has pledged a flat donation of $8 while his grandmother pledged $4 per mile. Using systems we could see: 8m+4g, g=8m/4. M represents the amount of his mom's pledges and g=the amount of miles Nigel is going will run. So since m=1 we'll just use g=8/4, which we know is 2. So, Nigel will have to run 2 miles for them to both give the same amount of money.
