Maths problem: how to do, thanks?

a) r = 16/40x15 = 6cm (similar to triangle proportion)

b) Use this Frustum total area formula: R=15, r=6, h = (40-16=24).  You may calculated the total surface area about = 2510 cm2.  

(a)__ 
　|\ 　\　　
　| .\ 　\　
　|16\ L' \　
　| . . .\　 \  
40|--r---\--- \ 
　|. . . . .\　 L 
　| . . . . . \　 \ 
　| . . . . . ..\　 \ 
　|___15__.\　 \ 

r/15 = 16/40
∴ r = 6 cm

(b)
smaller cone: L'²= 16²+6² = 292 => L' = √292
larger cone:  L²= 40²+15² = 1825 => L = √1825

lateral surface area of frustum 
= lateral surface area of (larger cone - smaller cone)
= π*15√1825 - π*6√292 = 1,690(to nearest 1 unit) - - - - - - - (#)
　　　　　　　　　　　　　 
area of upper & lower base = π*6² + π*15² = 820‬(to nearest 1 unit)

∴ total surface area of frustum = 1,690 + 820 = 2,510 cm²‬(to nearest 1 unit)

l agree with the claim that "total surface area of frustum is at least 2500 cm²"

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(#): lateral surface area of cone = πrL, where L=slant ht.