Just make 1 3/4 an improper fraction and multiply by 4:
1 3/4 = 7/4
7/4 * 4 = 7
Let m be the distance (in miles) she runs in a month
and w be the distance (in miles) she runs in a week
since the total distance she run in a week is one third of what she run in a month, in equation form
w = m/3
to calculate the distance she run in a month,
m = 3w
m = 3(6 miles)
m = 18 miles in a month