JULIA'S FIRST DAY AT SCHOOL
It’s the first day of school for Julia. The teacher, Miss Rivas, has told her students to meet in the playground. She says: “You must all shake hands and introduce yourselves”
If there were 10 students in her class, how many handshakes were there?
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The first student will shake hands with 9 students, the second student with 8, the third student with 7 and so on. So we have 9+8+7+6+5+4+3+2+1 =45
Or another way to solve the problem:
If there are 10 students, each one will give his/her hand to the other 9 students. That is 10 x 9 =90. However, if we calculate the handshakes this way, we are counting each handshake twice: for example when say B greets C and when C greets B. Thus the number of handshakes is half the calculations made above, therefore (10 x 9)/2=45