Elson finished reading a storybook last week. He read an average of $19$ pages per day for the first four days of the week. He read an average of $25$ pages per day from Friday to Saturday. He did not read on Sunday. How many pages on average did Elson read per day throughout the entire week? 
Overseas Competition->Knowledge Point->Counting Modules->Statistics and Probability
$(19\times4+25\times2)\div7=18$ pages. 