To write an equation for the total amount they spent at the restaurant, we need to add the bill and the tip. The tip is 18% of the bill, which means we need to multiply the bill by 0.18. The equation is:
total=bill+tiptotal=85.60+0.18×85.60
To find the total amount they spent, we need to simplify the equation by using the distributive property and then evaluate it:
totaltotaltotaltotal=85.60+0.18×85.60=85.60(1+0.18)=85.60×1.18=100.91
Therefore, they spent $100.91 at the restaurant.