The object's average acceleration over the 4s interval would be
[tex]a_{\rm avg}=\dfrac{50\,\frac{\rm m}{\rm s}-15\,\frac{\rm m}{\rm s}}{4\,\rm s}=8.75\,\dfrac{\rm m}{\rm s}[/tex]
Under constant acceleration, the average veclocity of an object with initial velocity [tex]v_i[/tex] and final velocity [tex]v_f[/tex] is
[tex]v_{\rm avg}=\dfrac{v_f+v_i}2[/tex]
so assuming "problem 4" refers to the first question, the average veclocity of the object would be
[tex]v_{\rm avg}=\dfrac{50\,\frac{\rm m}{\rm s}+15\,\frac{\rm m}{\rm s}}2=16.25\,\dfrac{\rm m}{\rm s}[/tex]
