Sentence 1: Chuck had just bought a new car stereo. 
 Sentence 3: So he called up his skilled friend for help 
 Sentence 4:  His friend came over and quickly installed the radio 
 Sentence 5:  Chuck was so happy, he gave his friend $20 to thank him! 
 Option 1: But he found that it was difficult to install! 
 Option 2: He installed quickly on his own.