Question 1203960
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You should understand the formal algebra that can be used to solve the problem, as shown in the response from the other tutor.  But I believe students will understand how to work problems involving arithmetic progressions better if they are able to work the problem informally -- which helps them understand WHY the formal algebra works.<br>
If we subtract the shortest thread length from the longest and divide the result by the difference between the lengths of successive threads, we get the number of threads AFTER THE FIRST ONE.<br>
(50.2-3.4)/1.2 = 46.8/1.2 = 39<br>
So the number of threads is 39+1 = 40.<br>
ANSWER: 40<br>