Explanation:
83 in burn in 5 hours.
so, we have a 83/5 speed ratio.
directly proportional means
y = kx
with k being constant for all values of x.
83 = k×5 = 5k
k = 83/5
15 in burn in x hours.
therefore,
15 = 83/5 × x
75 = 83x
x = 75/83 hours
this can be also calculated like this :
because of the direct proportionality, 15/x must have the same ratio as the original 83/5.
83/5 = 15/x
83x/5 = 15
83x = 75
x = 75/83 hours = 0.903614458... hours