First, you would need to convert 3/5 to a denominator of 10. So, multiply 3/5 x 2 which gives you 6/10. Finally, subtract 7/10 - 6/10 and you should get 1/10.
It is 1/10 of an hour. Since you can't subtract two fractions with different denominators, you need to get a common denominator. After multiplying the denominators and numerators to get the common denominator, you would get 35/50 and 30/50. Subtract 35-30 to get 5, keep the denominators. 5/50 is simplified to 1/10. Therefore, it is 1/10 of an hour