Statistics (random variable)

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A contractor is interested in the total cost of a project on which he intends to bid. He estimates that materials will cost rs 25,000 and that his labour will be rs900 per day. If he the project takes X days to complete,the total cost of the project (in rupees) will be given as:

c=25,000+900x

there is a table which gives the completion time, in days, for theproject.

Completion time X (days) : Probability
10 : 0.1
11 : 0.3
12 : 0.3
13 : 0.2
14 : 0.1

Find the mean and variance for completion time X.
For mean which is E(x) : i got 11.9
Variance V(x): i got 176.7

standard deviation (x), i got 5.92

Now find the mean and variance for total cost C.
C=25,000+900x

Mean= c=25000 +900x
E(c)=E(25000+900)
=(25000+ 900 v(x)
25000 + 900x
E(x) =11.9
=25000+900 x 11.9
=357,10

but am having problem to find the variance??

help pls!

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