Help help math math math math

If the wwight of a package is multiplied by 7/10 the result is 65.8 find the weight of the package

Can someone me out here and the tell me the greatest common factor

Jada has a meal in a restaurant she adds up the prices listed on the menu for everything they ordered and gets a subtotal of $42.00. when the check comes, it says they also need to pay $3.99 in sales tax. what percentage of the subtotal is the sales tax