Calculating Rule of 78 Loan Interest

SI [Interest] = (P x R x T) / 100
P [sum] = (SI x 100) / (R x T)
R [Rate/year] = (SI x 100) / (P x T)
T [Time] = (SI x 100) / (P x R)

where,
SI = Simple Interest
P = Principal or Initial amount
R = % Rate of interest per annum
T = Time Duration