how to delete columns from matrix

i have matrix (2,n) i want to delete the columns that contain a value < 20 from the two rows for exmple: a=[3 4 7 20 35 40; 29 70 5 90 45 2] the output a=[20 35; 90 45] i tried this code but not work correctly

if true
  % code
end
a=[3 4 7 20 35 40; 29 70 5 90 45 2]
for i=1:size(a,1)
for j=1:size(a,2)
if (a(i)<20) 
a(:,i)=[];
end
end
end

Answer 1

[David Fletcher]

a(:,all(a>=20))

[kmla]

thank you sir, this also work

    for i= size(a, 2):-1:1
a(:, a(1, :) < 20) = []
a(:, a(2, :) < 20) = []
end