Find the number of bicycles and tricycles

Without using equations and variables:

There are 3 times as many bicycles in the playground as there are tricycles.

Make groups of three bicycles and one tricycle each. Each group consists of 4 toys and has 9 wheels.

There is a total of 81 wheels.

There are 9 groups and thus 36 toys on the playground.

Hint: Let there be $b$ bikes and $t$ trikes. Each sentence provides an equation, giving two simultaneous equations in two unknowns. Or group three bikes with a trike (based on the first sentence). How many wheels does it have? How many groups are there?

Denote the number of bicycles by $b$ and the number of tricycles by $t$.

You know that $b = 3t$ from "3 times as many bicycles in the playground as there are tricycles." and you know that $2b + 3t = 81$ as the total number of wheels is the number of bicycles times $2$ (two wheels per bike) plus the number of tricycles times $3$ (three wheels per bike).

Now you can plug in $3t$ for $b$ in the second equation to get $2 (3t) + 3t = 81$ so $9t = 81$. From there you get $t$ and then $b$ by the first equation.

It is possible that your son is not really supposed to use more than one variable. In this case call the number of tricycles $t$ and argue that $2(3t) + 3t =81$ in about the same was as above.