David - you won't need a nested if statement, just an if and an elseif - the first to to check to see if the pipe weight is invalid, and the second to check if the pipe length is invalid. If either condition is true, then you would increment your number of rejects counter.
load pipe.mat;
numRejects = 0;
for k=1:size(pipe,1)
% check the weight
if pipe(k,1)<4.5 || pipe(k,1)>5.1
numRejects = numRejects + 1;
% check the length
elseif pipe(k,2)<15.3 || pipe(k,2)>15.4
numRejects = numRejects + 1;
end
end
