She smacked his growing cock. "Speak when spoken to. Don't make this weirder than it needs to be."
Cathy continued to stroke the growing member. "Hmm. How to approach this." She hopped on his legs and leaned in, her hair falling forward. She began by licking around the growing head. Then over the head, front to back and side to side. All the time Jim being silent, enjoying it. Once moist, she massaged under the glans. Her hand barely fit around but she managed. Loosening and tightening her grip, slowly rotating her hand around.
"You good up there?" she asked. Jim groaned in the affirmative.
"Good. Time for a math session." She put her mouth on the tip of his cock to wet it further. "Your girlfiend's height is 5ft 6 inches. How many inches is that total?" She started to stroke as she looked at him through her hair.
Jim's eyes opened and his mouth moved slowly. "56?" and his eyes closed as he moaned.
"Nope!" a smack on the balls at the same time brought his eyes open. "Try again."
"Uhhhh... 60ish?" with a moan.
"Closer." as she grabbed his balls and started kneading them. She ran her tongue around the swollen head. "Try again?"
"I'm a mage! We get people to do, eugh!" he started, and she clenched his testes. "66 inches!"
"Yes! Good. And ten percent of that is?" and she licked his shaft.
"6.6" he replied with his eyes closed, slowing raising his hips.
"Good. 66 minus 6.6?"
"Sixttttttyyyyy?" he groaned with the upwards stroke.
"No. Because you're close in two ways and currently very distracted, I'll tell you it's 59.4." said said with a downward stroke, pressing on the underside. "You're not allowed to let your girlfriend get less than that, right?"
"Uh huh. Yes ma'ammm" Jim moaned.
"Good. Glad we had that talk." Cathy then leaned in taking most of the head in her mouth. She started to pump slowly at first, then picking up speed. Her hand moved up and down in sequence with her mouth, then out of synch, then in. Jim's hips started to pick up speed, then slowed down as the pressure built.
Her motions continued for some time. Jim moaned and writhed with hips moving in time to her ministrations. All the while Cathy keeping him on the edge.
Finally Cathy moved to licking around the glans and reached up to play with his chest. That was the trigger he needed. Jim arched his back and Cathy greedily placed her mouth over the fountain and took it in. As Jim slowly pressed upwards, she moved with him and began glowing.
Jim finished and lay back. Cathy sat up and found a t-shirt hung over the foot of the bed. She used it to clean him and herself up. She looked down at the shirt she had on, noticing it fit better. Same with her shorts.
Cathy climbed off the bed which came under her waist now. She looked at Jim on the bed, nearly passed out. She leaned over, planted a kiss on his lips, "I'm going to my room to get cleaned up. We can have another math lesson tomorrow."
Jim nodded and lay back as she closed the door behind her.