This is a work/rate problem where we need to find the combined rate of two machines and then use that rate to determine the time needed to produce a certain number of items.

Step 1: Calculate the rate of Machine A.

Machine A produces 90 toys in 15 minutes.

$$
\(\text { Rate of Machine } A=\frac{\text { Number of toys }}{\text { Time }}=\frac{90 \text { toys }}{15 \text { minutes }}=6 \text { toys } / \text { minute }\) .
$$


Step 2: Calculate the rate of Machine B.

Machine B produces 60 toys in 30 minutes.

$$
\(\text { Rate of Machine B }=\frac{\text { Number of toys }}{\text { Time }}=\frac{60 \text { toys }}{30 \text { minutes }}=2 \text { toys } / \text { minute }\) .
$$


Step 3: Calculate the combined rate of both machines.

When running simultaneously, their rates add up.
Combined Rate $=$ Rate of Machine A + Rate of Machine B
Combined Rate $=6$ toys $/$ minute +2 toys $/$ minute $=8$ toys $/$ minute .

Step 4: Calculate the time required to produce 400 toys.
We know the combined rate and the total number of toys to be produced.

$$
\(\begin{aligned}
& \text { Time }=\frac{\text { Total number of toys }}{\text { Combined Rate }} \\
& \text { Time }=\frac{400 \text { toys }}{8 \text { toys } / \text { minute }}=50 \text { minutes. }
\end{aligned}\)
$$


So, it would take 50 minutes to produce a total of 400 toys when both machines run simultaneously.