Let
x--------------> the third stone measure
we know that
the recommended height for the bench is ------> 20 in
20=[4 3/8]+[4 1/2]+[5 1/4]+[x]
4 3/8---------> (4*8+3)/8=35/8
4 1/2---------> (4*2+1)/2=9/2
5 1/4---------> (5*4+1)/4=21/4
20=[35/8]+[9/2]+[21/4]+[x]
x=20-(35/8)-(9/2)-(21/4)
x=20-(35+4*9+2*21)/8----->
20-113/8------- > (8*20-113)/8
x=47/8----------
> 5.875=5 7/8
the
answer is 5 7/8 in
