If somebody knows a little about fluids can they please help...

Can someone explain clearly where these numbers are coming from and some advice on mathematics tools I need to solve similar manipulation of problems questions, this would be much appreciated....

Using continuity, U1 X A1 = U2 X A2 therefore u1 = u2 x 50sqr /100sqr = U2/4

So substituting back into Bernoulli's equ:

p1/row x grav + u1sqr/2 x grav = u2sqr / 2 x grav the other variables have been cancelled due to the actual problem.

We get pressure 1 or P1/ rough x gravity + U2 sqr/32 = u2sqr / 2 x grav and so p1/row x grav = 15xu2sqr / 32 x grav.

Can anyone help? Where does this 15 come from?