Assume that transaction costs are zero and there is no credit risk in any transaction. If the price at time 0 of a 2-year coupon bond with face value $1000 and coupon rate of 5% is $900 and the (forward) price at time 1 of a 1-year zero coupon bond with face value $1000 is $880, what must be the time 0 price of a 1-year zero coupon bond with face value $1000 in the absence of arbitrage opportunities?

Supposed to answer this by matching cash flows. Can anyone explain this concept? Thanks