NOT HAPPY WITH OUTCOME

we were involved in an accident. Not our fault, but in the email sent to us we are liable for 20% of his damages. We were travelling on a road and he came out of a complex. He took off while plugging in his adaptor for his cellphone. We are liable because we did nothing to prevent it, they say but he had earphones in his ear and did not hear us hooting. We are also said to have been speeding, we were doing 40km/h, as we had just come out of our complex. How then can we be liable?