Q20. Its a problem of Arbitrage....so very easily you find out one thing: 5s1-s2=3 & s1+s2=1...solving both eqns you get s1=2/3,s2=1/3.....for the next one 3s1+0.s2=3*2/3=2.
Q33. If the person 1 lexicographically prefers y to x...then getting his endowment bundle, he doesn't interested to trade with other...so the price of Py will be quite higher than the Px which is easily trade-able.....px/py<=1.
Q43. Ls=Ld when market is clear...-a +d.b=b=>b=a/d-1=>b>a/d
44.Not sure