A mercury barometer of height h has an imperfect vacuum above its mercury column so
that it measures a surface pressure of 29.80 inches Hg when the true surface pressure is
29.90 inches Hg, and it measures a surface pressure of 29.72 inches Hg when the true
surface pressure is 29.80 inches Hg.

What will this barometer measure when the true pressure is 29.7 inches Hg?

This question seems so easy yet I just cannot come up with an answer. I've been using the p=rho*g*z formula to try to set up a ratio but I keep ending up with two unknowns.