If y varies inversely as x and y =16 when x=5 find y when x=20

Answer 1

For Y to vary inversely with X means that when Y is doubled, X is halved. Or to put it into a formula

X*Y = Z, in this case Z = 80, so X*Y always equals 80

Therefore if X = 20, that means 20*Y = 80

20Y = 80

Divide both sides by 20

Y = 4

For another way to think about it, you can see that X increased from 5 to 20, so if X became 4 times (20/5) larger, then Y must become 4 times smaller. 16/4 = 4 = Y.