CastorPrep

The AI tutor

Castor — a tutor that's already worked the problem

Grounded in verified worked solutions and the NCEES Reference Handbook — the same document you'll have in front of you at the testing center.

How it works

Grounded, start to finish.

Stage 1

An authored problem

Every problem in the bank is original and written to the exam's own blueprint — the same areas, the same depth of calculation, the same Handbook dependence you'll face on exam day.

Stage 2

A verified worked solution

Before a problem goes live it carries a worked solution — the governing relation, the intermediate values, and why each wrong option is wrong.

Stage 3

Castor answers from that solution

When you ask Castor anything, it teaches from that verified work — purpose-built on frontier AI, scoped to the problem in front of you.

What it does

Three jobs, done properly.

Explains your miss

“You picked B — here's the step where B diverges.” Castor names the specific error that produces each wrong option, because the authored solution records it.

Cites the Handbook — or tells you it can't

Every reference points at the section you'd flip to on exam day, so those references double as reps at finding the value under closed-book CBT conditions. Where a problem carries no verified reference, Castor says so rather than inventing a section number.

Talks you through when stuck

Before you answer, Castor works in coach mode: it probes your setup, checks intermediate steps, and hands out graduated hints — one nudge at a time — while the final call stays yours.

Castor— AI tutor

Coach mode, before answering — a composite wall section: find the temperature at one layer interface.

Where do I even start with this one?
Start by identifying what you're looking for and what you know. You have the two air temperatures, the total wall R-value, and one specific plane you need: the interface between the insulation and the gypsum board. The key insight: temperature doesn't drop uniformly through a composite wall — it drops in proportion to the R-value of each layer. So the first move is to identify which resistances lie between a known temperature and the plane you're asked about. Which end of the wall — indoor or outdoor — is closer to that interface?
The room air, I think — 70 is given and it's right next to the interface. But I'm not sure which R-values count.
Exactly right — the room air is your known reference point, and it's on the same side of the interface as the value you want. Now: what layers sit between the room air and that interface? Read the wall section from the inside out. Start at the room and move outward until you hit it. Which material layers do you cross?
Gypsum and the inside film, so 0.45 + 0.68. How do I turn that into a temperature?
Good. So you have R = 1.13 between the room air and your target interface. Now use the proportionality: T_interface = T_room − (R_layer / R_total) × ΔT_total where ΔT_total is the difference between the outdoor and indoor air temperatures. Plug in your numbers and solve.

A real coach-mode exchange with Castor, on a live problem from the bank. The final call stays yours.

What it won't do

Castor won't do your thinking for you. Before you answer, it coaches — it won't hand you the letter. It's scoped to the problem in front of you and your own work, because a tutor that answers everything is just a chatbot, and chatbots don't get you licensed.

Use it freely

On every problem, in every plan.

Castor is included in every plan — no meters, no per-question credits. Ask as much as your studying needs, on every problem in the bank.

Start practicing free

An email address is all it takes.