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Bill gets  224.44  British pounds about.  rounding 224.4375

Step-by-step explanation:

399 British pounds  = A + B + C + D

A: B = 2:9

C +D  =  2.5 * A

so...

9A = 2B

399 = A + (9/2)*A  +  2.5*A

399 = 8A

A = 399/8 =   49.875

B = (9/2)*A = (9/2) *49.875 = 224.4375

C + D = 2.5 *  49.875  =  124.6875‬  

1 number

if we use a graphing calculator, there is only one such number that satisfies the conclusion of the mean value theorem, that is, a number such that

where the right-hand side is the average rate of change of f on the interval [0.5,0.75], which is the slope of the secant line.

in the attached graph, the black line is the secant line which has its slope as the average rate of change of f on [0.5,0.75], while the orange line is the only tangent line to the graph of f in the interval that satisfies the mvt conclusion.