Answer: a house


Estimating using the total weight in grams
110 tonnes $\times$ 50 years = 5500 tonnes
                                  = 5 500 000 kg
                                  = 5 500 000 000 g
21.5 g $\approx$ 22 g (22 and 55 both multiples of 11)

5 500 000 000 $\div$ 22 = (55 $\div$ 22) $\times$ 100 000 000
                              = 2.5 $\times$ 100 000 000
                              = 250 000 000

The platinum occupies 250 000 000 cm$^3$ = 250 m$^3$
                                                           = 10 m $\times$ 5 m $\times$ 5 m



Estimating using the volume of 1 kg
21.5 g is approximately 1 cm$^3$
1000 g is approximately 1 cm$^3\times$50 = 50 cm$^3$
1000 kg is approximately 50 cm$^3\times$ 1000 = 50 000 cm$^3$
110 tonnes is approximately 50 000 cm$^3\times$ 110 = 5 500 000 cm$^3$
5 500 000 cm$^3$ = 5.5 m$^3$
So about 5.5 m$^3$ per year
5.5 m$^3\times$ 50 years $\approx$ 250 m$^3$
250 m$^3$ = 10 m $\times$ 5 m $\times$ 5 m