X is partly constant and partly varies as Y. X=15 when Y=3 and X=23 when Y=5. What is the value of X when Y=10?

Answer 1

X is partly constant and partly varies as Y ----> x = ky + c
X=15 when Y=3 ---> 15 = 3k + c
X=23 when Y=5 ---> 23 = 5k + c
subtracting them:
2k = 8
k = 4

put that back into 15 = 3k + c to find c
Now you have your complete equation and you can find x when y = 10