a lifeguard located 20 yd from the water spots a swimmer in distress

a lifeguard located 20 yd from the water spots a swimmer in distress. the swimmer is 30 yd from shore and 100 yds east of the lifeguard. suppose the lifeguard ran then swam in a direct line to the swimmer. how far east from his original position will he enter the water? /// i already know the answer is 40 yd,,, i just dont understand how to get the answer