If borrows 1CAD in Canada and saves in US, the company will gain 1CAD*(spot rate)*(1+interest rate in US)/(1 year forward rate) after 1 year, which is 1*0,9*(1+3%)/0.8=1.15875CAD. The principal and interest to be repayed will be 1CAD*(1+interest rate in Canada), which is 1*(1+4%)=1.04CAD. After repaying the 1.04CAD, the company will still have a positive profit of 1.15875-1.04=0.11875CAD. Thus we can make an arbitrage profit.