They carry about 20-25 per cent of $\mathrm{CO}_2$
They transport 99.5 per cent of $\mathrm{O}_2$
They transport about 80 per cent oxygen only and the rest 20 per cent of it is transported in dissolved state in blood plasma
They do not carry $\mathrm{CO}_2$ at all
Solution
Nearly 20-25 per cent of $\mathrm{CO}_2$ is transported by RBCs. It is caused by haemoglobin as carbamino haemoglobin. 70 per cent of $\mathrm{CO}_2$ is carried as bicarbonates.
About 97 per cent of $\mathrm{O}_2$ is transported by RBCs in the blood. The remaining 3 per cent of $\mathrm{O}_2$ is carried in dissolved state through the plasma.