Question 1081942
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<U>Mental solution</U>


<pre>
The average length of four sides is {{{60/4}}} = 15.


Now imagine 4 points in the number line, equidistantly placed with the step "d", which is the common difference of the AP.


Then the distance from the center to the fourth (right-most) point is 1.5d.


So, 1.5d = 24 - 15 = 9.   Hence,  d = 6.


Thus the sides are 24, 24-6 = 18, 18 - 6 = 12  and  12 - 6 = 6.


<U>Answer</U>.  The sides are 6, 12, 18 and 24 cm.

<U>Check</U>.   6 + 12 + 18 + 24 = 60 cm.
</pre>

Solved (mentally, without equations).