A bank credits interest on deposit using accumulation factors based on a variable force of interest. On 1 July 2026, a customer deposited 846 with the bank. On 1 July 2028, his deposit has grown to 1,099. Assuming that the force of interest per annum is a + bt (on 1 July 2026 we fix t = 0, unit = year) during the period, find the force of interest per annum on 1 July 2027 (in % p.a.)