Answer:
B
Step-by-step explanation:
We are given that <em>x</em> and <em>y</em> are functions of time <em>t</em> such that <em>x</em> and <em>y</em> is a constant. So, we can write the following equation:

The rate of change of <em>x</em> and the rate of change of <em>y</em> with respect to time <em>t</em> is simply dx/dt and dy/dt, respectively. So, we will differentiate both sides with respect to <em>t: </em>
<em />
<em />
Remember that the derivative of a constant is always 0. Therefore:

And by subtracting dy/dt from both sides, we acquire:

Hence, our answer is B.
Jenny's algebraic expression:
y = 33d
where y is the total number of posts
total tweets in 5 days
y = (33)(5)
y = 165 tweets
Antonio's expression:
y = 5d
total tweets in 5 days
y = (5)(5)
y = 25 tweets
Altogether they have posted:
165 + 25 tweets = 190 tweets in five days
4 x 0.90 = £3.60
£13.50 - £3.60 = £9.90
0.9 + 0.75 = £1.65
£9.90/£1.65 = 6
They bought 6 ice cream cones