Jon went to the candy store to buy some candies for a birthday party. He bought a total of 20 lollipops and chocolate bars, and a total of 35 lollipops and sweets. The number of chocolate bars and sweets are in the ratio of 2 : 7. How many lollipops did Jon buy?

Let's denote:

L = number of lollipops C = number of chocolate bars S = number of sweets

From the problem, we know that:

1. L + C = 20 (Jon bought a total of 20 lollipops and chocolate bars)
2. L + S = 35 (Jon bought a total of 35 lollipops and sweets)
3. C/S = 2/7 (The number of chocolate bars and sweets are in the ratio of 2 : 7)

We can solve this system of equations step by step:

Step 1: From equation 1), we can express C as C = 20 - L.

Step 2: Substitute C from step 1) into equation 3) to get (20 - L)/S = 2/7. Multiply both sides by 7S to get 7*(20 - L) = 2S. Express S as S = 7*(20 - L)/2.

Step 3: Substitute S from step 2) into equation 2) to get L + 7*(20 - L)/2 = 35. Solve this equation for L.

After solving, we find that Jon bought 15 lollipops.

Let's denote:

L = number of lollipops C = number of chocolate bars S = number of sweets

From the problem, we know that:

1. L + C = 20 (Jon bought a total of 20 lollipops and chocolate bars)
2. L + S = 35 (Jon bought a total of 35 lollipops and sweets)
3. C/S = 2/7 (The number of chocolate bars and sweets are in the ratio of 2 : 7)

We can solve this system of equations step by step:

Step 1: From equation 1), we can express C as C = 20 - L.

Step 2: Substitute C from step 1) into equation 3) to get (20 - L)/S = 2/7. Solve this equation for S to get S = 7*(20 - L)/2 = 70 - 3.5L.

Step 3: Substitute S from step 2) into equation 2) to get L + 70 - 3.5L = 35. Solve this equation for L to get 2.5L = -35, so L = -35 / 2.5 = 14.

So, Jon bought 14 lollipops.