Can you me? this is systems with elimination

Nnnnneeedd 1.find the residual if you know the actual number is 5.2 and the predicted value is 4.8

During a bike challenge riders have to collect various colored ribbons each 1/2 mile they collect a red ribbon each eighth mile they collect a green ribbon and each quarter mile they collect a blue ribbion wich colors of ribion will be collected at the 3/4 markrer

Given the equation f(x)=a(b)x where the initial value is 40 and the growth rate is 20% what is f(4)