Try this: he made $450K in revenue by sending 75% of his time seeing patients; if he raises that time to 90% he can make (90/75) x $425K = $510K in revenue. The increase in revenue is therefore $510K - 425$ = $85K.
If you didn't divide by 0.75 the increase would be commensurate with increasing his time seeing patients from 75% to 75% x 1.15 = 86.25% of his time.
If it's still unclear, another method is to assume he works a 40 hour week. Currently he sees patients for 75% x 40 hours = 30 hours/week and spends 15% x 40 = 6 hours on billing. If he increased his patient time to 90% =36 hours, then the incerase in revenue is 6/30 x $425 = $85K.