If each math book is 0.75 of a inch wide you would do 18 divided by 0.75= 24. 24 books width can fit into the 18 inches.
Laura paddles in her canoe at a speed of 5 mph for 5 miles.
As we know that

Substitute the values we get

Laura hops on her windsurfer and sails for 12 miles at a speed of 6 mph.
Again using the above formula we can write

Hence from equation (i) and (ii), we can say total time of travel
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Formula
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Area of circle = πr²
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Find radius
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πr² = 75
r² = 75 ÷ π
r² = 23.87
r = √23.87
r = 4.89 cm (nearest hundreds)
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Answer : radius = 4.89cm
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19 because 1 ten = 10 ones and 9 one plus 10 ones equals 19 altogether